50 MORPHOLOGY OF 



The muscles of the third body cavity are not markedly 

 different from those of the collar. In B. Kowalevskii alone 

 a large muscular band runs along each side of the ventral 

 nerve-cord, forming a projection from the body [v. fig. 108). 



The perihsemal cavities are similarly almost filled with 

 tissue, and always contain more or less longitudinal muscle- 

 fibre. These are gathered into two bundles, and are inserted 

 into the notochord sheath in the proboscis stalk. They are 

 most developed in B. minutus, &c. (v. figs. 67 and 68). 



The Mesenteries. — The dorsal mesentery persists thi'ough- 

 out life in B. Kowalevskii and B. salmoneus. In the 

 other species it disappears in the collar region. The 

 ventral mesentery persists in the trunk in all species, but 

 is always obliterated in the collar. 



In B. minutus the body cavity of the trunk in the hepatic 

 region is again divided in consequence of an attachment 

 between the lateral angles of the diamond-shaped intestine to 

 the body wall {v. fig. 93). In this position two large lateral 

 vessels run. 



As Spengel has stated, strands of connective tissue run in 

 B. minutus from the body wall between the follicles of the 

 ovaries, forming a sort of radial septa. These septa are pro- 

 bably not of morphological importance, beyond indicating the 

 " accidental" way in which such septa may arise (cf. Poly- 

 gordius, &c.). 



All the body cavities are full of corpusculated fluid, as Spengel 

 has observed. These corpuscles, when living, are full of bright 

 granules and vacuoles, and exhibit amosboid movements. 



The Proboscis Gland. — In B. Kowalevskii (fig. 47, 

 gls.), at about the age of two gill-slits, a space appears in the 

 proliferation of mesoblast lying dorsal to the anterior end of 

 the notochoi'd, when the latter is pushed forwards into the 

 anterior body cavity. This space is the first rudiment of the 

 sac of the proboscis gland. Soon after its appearance it be- 

 comes enclosed in a membrane, which is added first at the 

 posterior part of the sac (cp. figs. 45, 31, and 47). Its cavity 

 is therefore a tissue space arising in the wall of the body cavity, 



