ON DEVELOPMENT ETC. OF PHOIIOXIS. 545 



belonging; to type C. But the position of tliese glands in rdatidii 

 to the ganglion as well as their histological structure at once 

 reveal their true nature. The internal openings of the organs 

 were described bv Masterman as follows : " Just ivherc the 

 preoral mesoblastie ivall slopes away on rlther side of the slnnx 

 there are a pair of thickenings, which traced forwards, show 

 themselves to be the commencement of a pair of internal openings^' 

 {I.e., p. 307). The paired thickenings referred to by him are 

 apparently nothing else than the points of attachment of the 

 retractor muscles in the collar cavity, as will be seen in fig. 59 d 

 {ret.). Further details respecting these muscles will be given 

 later. 



Larval Collar Cavity. The collar-cavity is a comparatively 

 wide space extending between the preoral and the postoral septa. 

 It is produced anteriorly into two horns, embracing between them 

 the posterior recess of the preoral cavity. It is perfectly separated 

 by the postoral septum from the trunk cavity. The postoral sep- 

 tum, or simply the septum, as it is more commonly called, is 

 stretched obliquely transversely between the splanchnic and the 

 somatic walls, along a line a little l)elow the tentacular circle (figs. 

 45, 48, mes^. Its dorsal attachment on the splanchnic layer is, as 

 represented in fig. 45 (mes.), found at the plane of the junction 

 of the œsophagus with the stomach, while ventrally the attach- 

 ment lies much further below. In frontal sections of the larva, 

 the septum (fig. 48 mes^ is seen on either side of the stomach 

 and its somatic insertion lies just under the tentacles, so that each 

 tentacular cavity is continuous with the larval collai- cavity (fig. 45). 



The adult collar cavity, or the supraseptal ca\'ity, is already 

 formed in the fully developed larva of every type, as a ring-space 

 runnino" alons; the inner side of the tentacular circle and above 



