Respiration in scoliodon or shark

Respiratory System of Scoliodon

🦈Scoliodon is an aquatic animal. 

🦈It depends completely upon oxygen dissolved in sea water for respiration. 

🦈Thus, respiration is aquatic and carried on entirely by vascular gills.

Respiratory Organs:

🦈There are five pairs of gill- pouches bearing gills, arranged in a series behind the hyoid arch in the lateral walls of the pharynx. 

🦈 Each gill-pouch is compressed antero-posteriorly and communicates with the cavity of the pharynx through a large internal branchial aperture and with the exterior through a narrow external branchial 

aperture also  called gill- slit.

🦈Two adjacent gill-pouches are completely separated from each other by a vertical fibro-muscular partition, the interbranchial septum or gill septum.

🦈The inner or pharyngeal border of each gill septum is supported by a cartilaginous visceral arch or gill arch with its slender branchial rays. 

🦈The septum is covered by an epithelium and contains blood vessels, nerves, etc.

🦈The mucous membranes of a septum are raised into numerous horizontal leaf-like folds, called gill lamellae or gill filaments. 

🦈These constitute the gill proper and are richly supplied with bloodcapillaries.

🦈 Each septum bears two sets of gill-lamellae, one on its anterior face and the other on its posterior face. 

🦈Each set makes a half gill called demibranch or hemibranch, while both the sets attached to a gill arch and its gill septum constitute a complete gill called holobranch. 

🦈 In Scoliodon  the interbranchial septa extend well beyond their gill lamellae to form flaps which protect the gills as well as external gill slits. Such gills are called lamelliform. 

🦈In  front of hyoid arch or the first internal gill slit, on either lateral side of pharynx, there is an oval pit of spiracle. 

🦈It has no gill lamellae and 'no external opening and is regarded a vestigial gill pouch.

Mechanism of respiration

🦈During respiration, water taken into the mouth, passes through internal gill slits bathing gill lamellae and passes out of the external gill slits.

1. Inspiration:

🦈 The floor of the buccopharyngeal cavity is depressed by the contraction of hypobranchial (hypoglossal) muscles due to which the visceral arches expand the wall of the pharynx, so that sea-water containing dissolved oxygen rushes in through the opened mouth. 

🦈Entry of water into external branchial apertures is prevented by an anterior fold of skin on each gill-pouch.

🦈 As the external gill slits are tightly closed, water enters the enlarged buccopharyngeal cavity through the open mouth

2. Expiration:

🦈During expiration the mouth becomes closed by the action of adductor muscle.

🦈 At the same time the constrictor and inter-branchial muscles contract raising the floor of pharynx and reducing its volume. 

🦈As a result, water is forced into gill pouches, over the gill lamellae, and out through the open external gill slits.

🦈 The spiracles are occasionally used as accessory pathways for entry of  water for respiration, instead of the mouth.

Physiology of respiration: 

In the branchial lamellae the blood flows from the tip towards the base, that is in a direction opposite to that of the water current, so that the blood just before leaving the lamellae meets the highest concentration of oxygen and the lowest of carbon dioxide, thus, an efficient exchange of oxygen and carbon dioxide takes place between the blood and sea-water.Sea-water entering the gill- pouches with the respiratory current contains oxygen dissolved in it. This water is separated from the blood 

contained within the capillaries of the gill-lamellae merely by the thin and permeable membranous walls of the capillaries. The oxygen of the water passes by endosmosis through the thin capillary walls into the blood, and at  the same time the carbon dioxide of the blood passes into the water by a process of exosmosis.The oxygen is conveyed by the blood to all the parts of the body, while carbon dioxide brought to the gills in the venous blood is eliminated by the water of the outgoing respiratory current. As the blood makes a complete circuit in the capillaries of the gills in a very short time, it is evident that exchange of gases also takes place very quickly

Digestive system of scoliodon

🦈The digestive system consists of the alimentary canal and the digestive glands.

 🦈The alimentary canal 

starts with the mouth and terminates in the cloaca. 

🦈The mouth leads into a spacious buccal cavity which 

is lined with mucous membrane.

🦈The floor of the buccal cavity becomes folded to form a non-muscular and non-granular ‘tongue’. 

🦈The mucous membrane is very thick and rough due to the presence of dermal denticles or teeth. 

🦈The teeth are very sharp and are obliquely placed . 

🦈The teeth are homodont i.e., the teeth are similar in shape and polyphyodont ie posesses several sets of teeth functioning in succession.

 🦈The buccal cavity leads into pharynx. 

🦈On either side of the pharynx there lie the internal openings of  the spiracles and five branchial clefts. 

🦈The mucous membrane of the pharyngeal wall contains 

numerous dermal denticles. 

🦈The pharynx leads into a narrow oesophagus. 

🦈The inner mucous membrane of the pharynx is raised to form longitudinal folds.

🦈 The oesophagus dilates posteriorly to form a large stomach. 

🦈The stomach is highly muscular and is bent on itself to form a J-shaped configuration. 

🦈The long limb of the stomach is continuous with the oesophagus and the shorter one passes into the intestine. 

🦈The entrance of the oesophagus into the 

stomach is provided with a crescentic fold which serves as the valve.

 🦈The long anterior limb is called the cardiac stomach and the short posterior limb is designated as the pyloric stomach. 

🦈A small outgrowth often called ‘blind sac’ is present at the junction of the cardiac and 

pyloric limbs.

🦈The inner lining of the cardiac stomach is folded longitudinally like that of oesophagus  

🦈The internal lining of the pyloric stomach is mostly smooth though slight foldings are observed at the distal end.

🦈The pyloric valve, at the end of the pylorus, guards the entrance of it into a thick-walled small  chamber called the bursa entiana. 

🦈The bursa entiana is immediately followed by wide tubular intestine which becomes narrowed posteriorly as the rectum. 

🦈The rectum opens into the cloaca. 

🦈A tubular caecal or rectal or digit form gland opens into the rectum.

🦈The inner surface of the intestine becomes folded to form an anticlockwise spiral of approximately two and a half turns. 

🦈This is called the scroll valve which increases the absorptive surface of the intestine and also checks the rapid flow of digested food through the intestine.

🦈The major digestive gland is the liver which is a massive yellowish gland and consists of two lobes. 

🦈The lobes are united anteriorly. 

🦈A thin-walled V-shaped gall-bladder is present in the anterior part of the right lobe of liver. 

🦈The bile duct receives a few smaller ducts from the two lobes of the liver and opens 

into the anterior end of intestine near the commencement of the scroll valve.

🦈The pancreas is a pale compact irregular body and consists of a dorsal lobe situated parallel to the 

posterior part of cardiac stomach and a ventral lobe which remains closely attached to the pyloric stomach. 

🦈The pancreatic juice is poured into the intestine by pancreatic duct situated opposite to the aperture of the bile duct.

🦈The functional significance of rectal gland is not properly known. 

🦈The *rectal gland* has a central cavity lined with cuboidal cells. 

🦈It is highly vascular and composed of lymphoid tissue. 🦈It discharges a fluid 

into the lumen of the intestine but its actual role is not known.

🦈The buccal cavity possesses no salivary glands.

Lecture notes on reproduction, fertilization and development in penaeus (prawn)

Penaeus Female Reproductive System:

The female has a pair of long ovaries extending the whole length of the thorax and the abdomen along the median line. The two ovaries are fused together posteriorly, but free anteriorly. Near the anterior end the ovaries produce finger-like outgrowths called diverticula. From each ovary arises an oviduct.The oviduct opens to the third walking leg.

Penaeus_Male Reproductive System:The male has a pair of tubular testes located in the thorax on either side of the middle line. The two testes are fused together anteriorly.Each testis has many finger-like outgrowts called diverticula. Posteriorly, the testis leads into a vas deferens. The terminal end of the vas deferens becomes dilated into an ejaculatory bulb. The ejaculatory bulb opensbto the outside by the male genital pore at thevbase of the last walking leg.

Fertilization and Development:In penaeus  fertilization is external. During copulation, a male puts the female down on her back and deposits sperms (spermatophores) on the ventral surface   in the thelycumnear  the female genital pores. As the eggs come out of female genital pore, they are fertilized by the sperms already present there. The female lays eggs in large numbers. The fertilized eggs are attached to the pleopods by a sticky secretion of the tegumental glands. In Penaeus development is indirect. The fertilized egg hatches into a larva called nauplius. The nauplius is followed by a series of larval forms, namely metanauplius, protozoea, zoea and mysis. Finally the Mysis larva is transformed into a prawn.

1. NaupliusThe larva hatched from the egg is called nauplius larva. It has the following features:It is a free swimming minute, microscopic and pelagic larva. It is oval in shape; the anterior end is broad and the posterior end is narrow.The body has three regions, namely the anterior head, the middle trunk and the posterior anal region.The head has a simple median eye on the dorsal side called nauplius eye.It has 3 pairs of appendages. They develop into the first 3 pairs of appendages of adult prawn. The first pair of appendages is uniramous and is called antennule. The second and third pairs of appendages are biramous and are called antenna and mandible. The mandible has no teeth.The posterior end has a caudal fork.The nauplius larva grows and undergos several moults and finally develops into the next larva called metanauplius.

1. 2. 3. The three pairs of appendages of the nauplius larva (the future first and second antennae and mandibles).

3. Mandible

2. Metanauplius:It is the second larva of Penaeus. It has an anterior cephalothorax and a posterior abdomen. The abdomen ends in a caudal fork.The anterior end has a pair of frontal sense organs. The larva has three pairs of appendages, the next three pairs of appendages develop as buds.

3.Protozoea: Protozoea has an elongated body, divisible into cephalo-thorax and abdomen. Cephalo-thoracic region is covered by a dorsal shield or carapace. Theseven pairs of appendages of metanauplius become well developed. Alittle later, the 3rd pair of maxillipedes also develop. On the sides of the median eye a pair of lateral eyes appear.Protozoea grows in size, undergoes moulting and transforms to zoea.

4.Zoea:Zoea develops from the metanauplius.It is a free swimming pelagic, minute and microscopic.The body is divisible into an anterior cephalothorax and a posterior abdomen.The cephalothorax is covered by a carapace.Anteriorly the carapace is produced into median spine called rostrum.It has a pair of stalked compound eyes.All the 5 cephalic appendages are well developed.In the thorax, the 3 pairs of maxillipedesare slightly developed and the remaining 5 pairs of walking legs are in the form of buds.The abdomen has 6 segments. The last segment ends in a forked telson. The abdomen is without appendages.

5. Mysis:The mysis larva develops from zoea larva.It is elongated, laterally compressed and transparent.The body is divisible into a cephalothorax and an abdomen.The cephalothorax is covered by a carapace.Anteriorly the carapace is produced into a spine called rostrum.All the appendages are developed. They include 5 pairs of cephalic appendages, 8 pairs of thoracic appendages and 6 pairs of abdominal appendages.The thoracic legs are used for swimming. The abdomen ends in a telson.The eyes are stalked and compound.The mysis larva looks like the adult Mysis and hence the name. The larval mysis differs from the adult Mysis in the absence of a brood pouch beneath the thorax and a pair of statocysts.