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PROF. J. D. F. GILCHRIST ON A 



(5) Cervical groove. 



The absence of a cervical groove is said to be a distinctive feature of the 

 puerulus. This was found in the post-pnerulus stage, but not so well marked 

 in the dorsal region of the body as in the lateral regions, especially in front 

 of the two large anterior branchial spines. On microscopic examination it 

 was seen that this part was well defined by a line of low elevations of the 

 cuticle (fig. 3) which become the row of spines bounding the hepatic region 

 in the adult. 



(6) Mandibles. 



An interesting difference, and perhaps one of the most important, between 

 the puerulus and post-puerulus is found in the changes which have taken 

 place in the mandibles. These seem not only to distinguish the two stages 

 from each other, but also both from the adult condition. Thus, in the 

 puerulus stage, the mandibles are provided with a thick cuticle, which shows 

 no differentiation except three low prominences on the cutting surface. 



Fig. 10. — Mandible of post-puerulus. Fig. 11. — Mandible of puerulus. 



p., mandibular palp ; t. 1, t. 2, t. 3, teeth of mandible. 



The first and second of these, counting from the side next the mandibular 

 palp, are usually close together, though, in some cases, all three are about 

 equidistant. The third being situated somewhat internal to the other two, 

 though still forming part of the cutting-edge, I formerly suggested (4) might 

 prove to become the molar or grinding part of the adult mandible. An 

 examination of the post-puerulus does not, however, entirely confirm this. 



The changes which have taken place are illustrated by figs. 10 and 11, 

 which are camera lucida drawings of the shed cuticle of a puerulus and the 

 mandible of the post-puerulus stage, which takes its place, both from the 

 same specimen. The mandibular palp (p.) is now fully formed and is pro- 

 vided with three segments and seta?. The beginning of the mandibular 

 spine is also indicated but only by a clear line in the cuticle, which will form 



