VIII 



AETHEOPODA 



175 



the mid-ventral line, and it has been surmized that they are a last 

 remimseence of the ventral ciliated groove which extends between 

 mouth and anus in many Annelid larvae. 



In certain of the hinder segments of the body the dorsal divisions 

 of the coelom, after giving rise to the lateral walls of the heart and 

 to the pericardial septum, do not utterly collapse, but retain narrow 

 cavities. The somites belonging to several successive metaineres 

 iuse with one another so as to form two longitudinal tubes which 

 constitute the genital organs (Fig. 125 D). In the penultimate 

 segment the division of the somite into dorsal and ventral portions does 

 not take place; the excretory organ which belongs to this segment 

 forms the lower portion of the genital duct, whilst the upper portion 

 of the same duct is formed by the undivided coelom belonging to 

 that segment. 



Pig. 126. — The formation of the appendages in the embryo of Peripaius capensis. 

 (After Sedgwicli.) 



A, side view of an embryo in wliicb tlie appendages are in process of formation, to show the ventral 

 concavity and the dorsal hump. B, ventral view of the head of a much older embryo, to show the 

 cerebral groove and the lip which surrounds the buccal cavity, at, antenna ; eg, cerebral groove ; gn, 

 gnathite (jaw) ; gn.h, swelling at base of jaw ; I, lip enclosing buccal cavity ; or.p, oral papilla. 



It remains to be added that a stomodaeum and proctodaeum 

 are formed by ingrowths of ectoderm round the lips of the original 

 mouth and anus, which displace these openings inwards ; that on 

 the under side of the ectodermal thickenings, which give rise to the 

 brain, two deep pits are formed termed the cerebral grooves {eg, 

 Fig. 126 B) which later become closed off from the exterior, and form 

 for a time hollow appendages of the brain ; that the outer buccal 

 cavity which envelops the jaws is formed by the growth of a semi- 

 circular fold {I, Fig. 126 B) ; that the salivary glands are the excre- 

 tory organs belonging to the jaw segment, from the tubes of which 

 glandular pouches grow out, which project backwards ; and finally. 



