to the mollusca by Forbes, and even to the vertebrates by Meiss- 

 ner. Its development and structure, however, show that it is 

 nearly related to the Nematodes. The mouth is, however, armed 

 with six pairs of bristles ; and a double-fin-like expansion of the 

 sides and ends of the body gives it a slightly fish-like shape. This 

 fin-like expansion is seen in the (Vivaria, an- 1 the young ascid- 

 ian, and is of little morphological importance. It swims on the 

 surface of the water, not seeking the bottom or living parasiti- 

 cally. 



Development of Sagitta. This animal is a hermaphrodite, and 

 the eggs may be found in August well developed. Its develop- 

 ment has been studied by Gegenbaur and m m 

 Kowalevsky, by the latter in great detail. 

 The egg undergoes total segmentation, a 

 segmentation cavity being formed and the 

 blastoderm invaginating exactly as in the 

 Nematodes. This results in the formation 

 of a gastrula-condition (Fig. 179) in which 

 the infolding of the blastoderm leaves a 

 well marked primitive body cavity. Soon 

 at the opposite end of the body another 

 cavity (the permanent mouth) forms, which deepens and coi 

 with the primitive body cavity ; this closes up at the posterio 

 and the true digestive canal is formed. The embryo is ova 

 soon elongates, and the adult Sagitta form is attained befoi 



The phases of development are then as follows : 



2. Gastrula (well marked, but not ciliated and free). 



3. Adult Sagitta. 



IV. ACANTHOCEPHALI. 

 The Echinorhynchus (Fig. 180, head, after Owen) ; 181, the 

 muscles; cc, retractile muscles; from Owen), a singular worm, 



