KNTEROPNEUSTA 879 



opens to the outside at its posterior end, the cells of its walls 

 merging into the epithelial cells of the body-wall. Similar 

 communications of the inner space are also said to exist at 

 the anterior end (neuropore according to Bateson). A stout 

 nerve, which extends along the entire dorsal mid-line of the 

 body, is given off from the central organ. This in turn gives 

 off two nerves just behind the collar, which extend down- 

 wards (venti'ad), unite in the region of the first pair of gill- 

 pockets, and extend backwards in the body as the ventral 

 median nei^ve (Fig. 166 vn). 



The genital organs of Balanoglossus either belong to the 

 branchial region of the body or lie behind this. Balanoglossus 

 is dioecious. Male and female organs are entirely alike as 

 regards form and position. The genital glands lie in the 

 form of simple or branched tubes on both sides of the body, 

 and their external openings are found one behind the other, 

 forming two rows on the dorsal surface (Fig. 166 g). In 

 addition to these lateral rows of sexual organs (Fig. 164 g), 

 two others (median) lying between the gill-pockets and the 

 dorsal blood-vessel may make their appeai-ance. In many 

 species the part of the body succeeding the gill-pouches can 

 also be called the genital region, for the sexual organs are 

 especially well developed there. Owing to the fact that the 

 parts which contain the sexual glands undergo a great flatten- 

 ing and lateral extension, wing-like extensions of the body are 

 produced in certain species, e.g. B. claviger and B. inimihix, 

 studied by Kowalevsky. 



Development without the Tornaria Larva. — The 



fertilization of the eggs takes place outside the body in the 

 sea-water, into which in the Ameincan species studied 

 by Bateson (Balanoglossus Koivalevskii) both kinds of 

 sexual products are said to pass by the rupturing of the 

 body-wall. Artificial fertilization could not be under- 

 taken, though Bateson found the eggs in large quantities in 

 the slimy sand inhabited by the adult animals. The eggs 

 are closely enveloped by a delicate membrane, which sepa- 

 rates from the egg when fertilization has taken place. 

 Cleavage is total and tolerably equal. A blastula arises as 



