no COMPARATIVE ANATOMY 



or three fine teeth. The two pairs of antennse show little 

 change, the second pair continuing to be of large size, and 

 acting as the principal locomotory organs. The excretory 

 organs at their bases are distinct and functional. The shell- 

 gland, of which traces could be seen even in the earlier stage, 

 is now clearly visible in the dorsal shield, but it should be 

 noted that the shield extends only a little way backwards. 

 At this stage it scarcely overlaps more than the two first 

 thoracic segments. In the interior of the body the heart, 

 extending as far back as the sixth thoracic segment, has made 

 its appearance. 



After the third ecdysis many of the more important organs 

 of the adult are definitely established, but the larva is still far 

 from resembling the adult animal, chiefly because of the still 

 preponderant size of the second antenna. The body is further 

 elongated by the addition of new segments behind those 

 already existing : the caudal styles are much longer, and 

 twelve pairs of thoracic limbs are present, the first seven pairs 

 furnished with exites as well as endites. The cephalic shield 

 extends backwards to cover the third or fourth segment. The 

 paired eyes are being formed behind and above the median 

 larval eye, and beneath them the rudiment of the cerebral 

 ganglion can be distinguished, sending forward a pair of nerves 

 to - the frontal tentacles. The digestive glands are being 

 formed as simple finger-shaped outgrowths from the anterior 

 part of the gut, and the shell-glands have advanced so far in 

 development that they have nearly attained their adult 

 character, and receive a blood supply from a pair of lateral 

 arteries given off from the anterior end of the heart. The 

 heart itself extends as far back as the ninth thoracic segment, 

 and the circulation of the blood is established. The ventral 

 nerve chain can be distinguished reaching back to the ninth 

 thoracic segment. The second pair of maxilte is only repre- 

 sented by a small conical process bearing a single bristle, but 

 the first maxillae are fairly advanced, and their anterior biting 

 lobes are differentiated. The first and second antennae have 

 not undergone much change, but the mandibles are losing 

 their original character of swimming limbs and are becoming 

 powerful toothed biting organs, of which the distal biramose limb 

 is little more than an appendage. At each successive ecdysis 

 the larva becomes progressively more and more like an adult 



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