304 



EMBRYOLOGY 



Nereida, however, the hinder part of the body, thus equipped, does not 

 become detached, but its better equipment serves only to facilitate the 

 locomotion of the sexually mature animal (Ehlers). But in any event 

 the conditions existing in the Nereidce and Syllidce are comparable with 

 each other. 



Under the influence of special conditions of life the reproduction of 

 the Syllidce may assume a very peculiar form. In the sponge, Aulochone, 

 and other Hexactinellids a Syllis has been found (McIntosh), on which 

 new individuals arise, not only one after another, but also by lateral 



budding {Syllis ramosa, Fig. 141). 

 A genuine stock is formed in this in- 

 stance, the branches of which extend 

 without limit in the canal system of 

 the sponge, for the branches in turn 

 have the power of producing new 

 buds. These detach themselves from 

 the stock as male and female sexual 

 animals (Fig. 142) ; and since they 

 are provided with better swimming 

 apparatus, and with especially well 

 developed eyes, one can readily as- 

 sume that they abandon the sponge, 

 and, swarming about free in the sea, 

 secure a wider distribution of the 

 sexual products. Their descendants, 

 which they produce by sexual means, 

 then migrate back, it is to be as- 

 sumed, into sponges. In this in- 

 teresting case the alternation of 

 generations combined with stock-formation is particularly evident. 



Fig. 142.— Anterior part of a female 

 individual, such as is found in the 

 sponge stocks inhabited by Syllis 

 ramosa. The animal is filled with 

 eggs. The large eyes can be recog- 

 nized on the head (after McIktosh). 



II. ECHIURID>qE. 



JSchiurus ; Thalassema ; BonelUa. 



While BonelUa viridis lays its eggs in the form of a thick, 

 tortuous cord, consisting of a gelatinous mass, in which the 

 eggs lie in several rows (Spengel), the Thalaasema mellita 

 observed by Conn discharges its eggs and spermatozoa free 

 in the sea, so that with this animal artificial fertilization 

 could be undertaken. 



1. Cleavage and Formation of the Germ-layers. 



The cleavage of the egg was closely studied by Spengel in 

 BonelUa. An animal portion of the egg, consisting of finely 



