572 



CLASS II. ONYCHOPHORA. 



side, and forms a tube the generative tube (Fig. 345 D, 2). 

 The last section of this tube retains its connexion with the 

 ventral portion of the somite, and so acquires an external open- 

 ing, which is at first lateral, but soon shifts to the middle line 

 and fuses with its fellow, to form the single generative opening. 

 The praeoral somite develops the rudiment of a nephridium, but 

 eventually entirely disappears. The jaw somite also disappears 



FIG. 345. A series of diagrams of transverse sections through Peripatus embryos to show 

 the relations of the coelom at successive stages (after Sedgwick). A, early stage : 1 gut ; 

 2 mesoblastic somite ; no trace of the vascular space ; endoderm and ectoderm in 

 contact. B, endoderm has separated from the dorsal 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 unimportant. The dorsal part of each somite has 

 travelled dorsalwards, and now constitutes a small space (triangular in section) just dorsal 

 to the gut. The ventral portion (2') has assumed a tubular character, and has acquired 

 an external opening. The internal vesicle is already indicated, and is shown in the 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 fcirth ; 

 numbers as in C, except 5, slime glands. The coelom is represented as surrounded by a 

 thick black line, except in the part which forms the internal vesicle of the nephridium. 



the somites of the oral papillae form ventrally the salivary glands, 

 which are thus serially homologous with nephridia. The peri- 

 visceral cavity of Peripatus is, as in all Arthropoda, a haemocoele. 

 Its various divisions develop as a series of spaces between the 

 ectoderm and endoderm, and later in the mesoderm. The 

 mesoderm seems to be formed entirely from the proliferation of 

 the cells of the mesoblastic somites. It thus appears that in 



