io8 COMPARATIVE ANATOMY 



that its internal organs can scarcely be distinguished; the 

 most obvious organ is the gut, which is a relatively wide, 

 straight tube extending the whole length of the body, and 

 swollen at its anterior end into a right and left prominence, 

 the primordia of the digestive glands. The rudiments of 

 new organs are gradually formed beneath the cuticle, but they 

 do not become manifest until the larva has cast off its chitinous 

 cuticle, which it does by a succession of moults called ecdyses. 

 After the first ecdysis the larva is more elongated and has 

 completely lost its oval shape. The three pairs of nauplius 

 appendages have not undergone any material change except 

 that the basal joint of each mandible bears a large masticatory 

 process as yet unprovided with teeth. The second antennae 

 are still very large, and the most remarkable thing about them 

 is the development of a coiled tubular excretory gland opening 

 at the base of each. These are the larval excretory glands, 

 which eventually disappear, and are replaced by the adult 

 excretory organs or shell-glands. Their position is noteworthy, 

 for they occupy the position of, and are no doubt homologous 

 with, the permanent excretory organs of the higher Crustacea 

 e.g. the crayfish. The first antennae have undergone no 

 change of note, but in front of them, just behind the unpaired 

 larval eye, there is a small pair of tentaculiform projections 

 which in all probability represent the sense organs of the 

 pre-oral lobe or prostomium, and therefore may be considered 

 homologous with the cephalic tentacles borne on the pro- 

 stomium of so many Annelids. The first pair of maxillae, 

 which were not represented in the nauplius, are now visible as 

 a pair of simple plate-like outgrowths behind the mandibles. 

 The altered shape of the body is chiefly due to the changes 

 which have taken place in the posterior or thoracico-abdominal 

 region. There are five well-marked segments, and behind 

 them indications of two or three more can be distinguished 

 beneath the cuticle. The three or four most anterior of these 

 segments bear limbs, in the shape of oblique ridges divided 

 into lobes at their free edges. These lobes represent the 

 endites. The posterior extremity of the body ends in a pair 

 of short but distinct caudal processes, between which the 

 anus is situated. After the second ecdysis the larva has 

 attained a length of about i mm., and has undergone further 

 changes. The thoracico-abdominal region has grown longer 



