246 



THE PROTOCHORDATA. 



Fig. 115. Diagrammatic transverse sec- 

 tion through hinder region of proboscis of 

 Balanoglossus. (From a drawing kindly 

 lent by Professor T. H. MORGAN.) 



D. Dorsal. V. Ventral, be 1 . Proboscis- 

 cavity, almost filled up by mesenchymatous 

 and muscular tissue,* proliferated from the 

 original ccelomic epithelial layer (indicated 

 by the black line below the ectoderm). 

 p.v. Pulsating vesicle. A. Heart, ch. Noto- 

 chord. n.s. Integumentary nerve-plexus. 



The genital organs, 

 testes or ovaries, accord- 

 ing to the sex of the 

 individual, occur as a 

 paired metameric series 

 of pouch-like bodies or 

 gonadic sacs which ex- 

 tend backwards far be- 

 yond the region of the 

 gill-slits. The gonadic 

 sacs are suspended in the 

 body-cavity by solid cords 

 attached to the dorsal 

 integument, which be- 

 come perforated in the 

 spawning season to ad- 

 mit of the expulsion of the 

 reproductive elements. 



Metamerism. 



Although there is no muscular metamerism in Balano- 

 glossus, yet we have seen that other organs (gill-slits and 

 gonads) are arranged metamerically. And in point of 

 fact, among those Invertebrates which are not included 

 under the phylum of the Articulata, if there is one pecu- 

 liarity of organisation more sporadic in its occurrence than 

 another, it is metamerism. It may affect the most differ- 

 ent organs of the body either collectively or individually, 

 and nothing is more patent than the fact that the meta- 

 meric repetition of parts has arisen independently over 

 and over again in different groups of animals. 1 



* This tissue is not represented in Figs. 114 and 116, although it is present 

 throughout the body-cavity. 



