PERIPATUS 



of the somites is shown in Tig. 14, A, wliich represents ;i transverse 

 section taken between the mouth and anus of an embryo of the 

 stage of Fig. 13, D. The history of these somites is an exceed- 

 ingly interesting one, and may lie descrilied shortly as follows : — 

 They divide into two parts — a ventral part, which extends into 



Fig. 14. — A series of diagrams of transverse sections througli Peripatus embryos to 

 show the relations of the coelom at successive stages. (After Sedgwic];.) A, Early 

 stage : 1, gut ; 2, inesoblastic somite ; no trace of the vascular space ; endoderni 

 and ectoderm in contact. B, Endoderm has separated Irom the doisal and ventral 

 ectoderm. The somite is represented as having divided on the left side into a 

 dorsal and ventral portion : 1, gut ; 2, somite ; 3, haemocoele. C, The haemocoele 

 (3) has become divided up into a number of spaces, the arrangement of which is 

 nninrportaut. The dorsal part of each somite has travelled dorsahvards, and now 

 constitutes a small space {triangular in section) just dorsal to the gut. The ventr.il 

 ])Ortion (2') has assumed a tubular character, and has acquired an external opening. 

 The internal vesicle is already indicated, and is shown in tlie diagram by the thinner 

 black line: 1, gut; 2', nephridial part of coelom; 3, haemocoele; 3', part of 

 haemocoele which will form the heart — the part of the haemocoele on each side of 

 this will form the pericardium ; 4, nerve-cord. D represents the conditions at 

 the time of birth ; numbers as in C, except 5, slime glands. The coelom is re- 

 jiresented as surrounded by a thick black line, except in the part which forms the 

 internal vesicle of the nephridium. 



the appendage, and a dorsal part (Fig. 14, B). The ventral part 

 acquires an opening to the exterior just outside the nerve-cord, 

 and becomes entirely transformed into a nephridium (Fig. 14, 

 IJ, 2'). The dorsal part shifts dorsalwards and diminishes rela- 

 tively in size (Fig. 14, C). Its fate differs in the different parts 



