18 the anatomy axd development of peripatus xovae-britanxiae. 



Ventral Organ axd paired Ectodermal Organs of the Anal Segment. 



Approximately in the same transvei-se plane with the pygidial orifice, I have 

 observed five shallow epidermal involutions, two dorso-lateral, two ventro-lateral and 

 one median ventral above the ductus ejaculatorius (Fig. 22). 



Their symmetrical disposition indicates that they are definite structures and they 

 bear a strong resemblance to the ventral organs. Moreover the median ventral involution 

 is, in fact, the ventral organ of the anal segment, and it occurs also in the female 

 behind the vulva, between the latter and the anus. Of the other involutions I have 

 only observed the ventro-lateral pair in the female where they occur in the same 

 transverse plane with the supra-rectal commissure. 



In Fig. 22 the section is taken slightly posterior to the supra-rectal commissure 

 immediately in front of the line of insertion of the free male papilla on to the body- 

 wall, and therefore in the male the ventral organ of the anal segment opens into the 

 angle formed between the penis and the body-wall. In sections through a late male 

 embryo these structures present more the appearance of ectodermal thickenings with 

 slight traces of involution exactly like the ventral organs. The occurrence of paired 

 organs in the anal segment possibly homodynamous with the ventral organ of the 

 same segment is a fact of some interest and importance. 



DEVELOPMENT. 



The amount and state of preservation of my material enable me to give an account 

 of the development of P. novae-britanniae complete enough to render intelligible the 

 principle according to which it takes place. As I have already mentioned, all stages 

 of development from the segmenting ovum to the fully formed and pigmented embryo 

 are to be found in one adult female. 



The first intimation which I received that there was anything remarkable about 

 the embryos of this species was from an examination of the embryo represented on 

 Plate III, Fig. 35. In this figure it is seen that the anterior extremity of the embryo 

 is not coincident with the insertion of the antennae, but there is a long process of 

 the body extending nearly to the tip of the recurved abdomen and covering over the 

 ventral surface of the embryo like a cap, or better still perhaps, like an amnion. It 

 is however not an amnion, but is the remains of a large embryonic vesicle which 

 promotes the nutrition of the embryo and may therefore be called a trophic organ. 



For convenience of treatment the development may be divided into twelve stages, 

 which I will at once enumerate. 



Stage I. segmentation stages. 



„ II. formation of blastodermic vesicle. 



„ III. embryonic area (including primitive streak) at hinder end of blasto- 

 dermic vesicle (Fig. 23). 



„ IV. caudal extension of blastodermic or trophic vesicle, so that the 

 embryonic area becomes removed from posterior end (Fig. 24). 



„ V. invagination to form the ventral surface (Fig. 26). 



„ VI. forward free growth of primitive streak (Fig. 27). 



