HOMOLOGIES. 145 



formed by the fleshy collar of the body, which projects after the manner of that in Terebella, 

 but without the break or fissure. 



In his excellent account of the anatomy of Balanoglossus clavigerus and B. minutus, 

 Kowalewsky describes the chief muscular fibres of the "proboscis" as longitudinal, and the 

 circular as insignificant. This does not quite agree with the state of the parts in the foregoing 

 examples. 



Structure of the Body-wall. 



Few traces of the cutaneous elements remain in any of the specimens, but the structure 

 of fragments in the furrows demonstrates that it is allied in the closest manner to that of the 

 Nemerteans. The cutis consists of a multitude of cells and globules in a gelatinous inter- 

 cellular substance, the skin on section being streaked and loaded with circular and elongated 

 granular masses, as in the former group. Indeed, the ease with which almost the whole 

 cutaneous elements had separated from the subjacent tissues corroborated the relationship. 

 A tough and continuous basement-membrane, having a finely streaked appearance, intervenes 

 between the former and the next coat, which is a thick layer of longitudinal fibres, most 

 developed, perhaps, on the ventral surface. The interfascicular substance is slightly marked, 

 but there are many intersecting fibres which radiate inwards from the outer margin of this 

 investment, through the next layer, to the wall of the digestive chamber. In longitudinal 

 sections the longitudinal coat has, therefore, a transversely streaked aspect. It also presents 

 three well-marked dorsal gaps anteriorly, viz. a median and two lateral, while ventrally a single 

 hiatus exists in the centre. The circular muscular coat, which comes next in order, is 

 moderately developed. The space between the latter and the wall of the digestive canal is partly 

 occupied by the divergent fibres previously mentioned, the glandular or " liver"-tissue, and a few 

 cells and globules. 



The examination of living specimens enabled Kowalewsky to see the cilia with which the 

 whole integuments are covered, and he further describes a fine " cuticula " but, so far as an 

 examination of preserved specimens warrants me in affirming, this structure is not more differen- 

 tiated than in the Nemerteans, and therefore not demonstrable histologically as a special layer. 

 In his anatomy of the body- wall he places the circular muscular coat to the exterior of the 

 longitudinal — beneath our basement-membrane, and thus his specimens deviate in type from 

 the foregoing. 1 



Within the circle formed by the collar a conical process having a filiform terminal append- 

 age projects from the truncated anterior extremity of the body, and fits into the hollow at the 

 base of the " proboscis." This structure is supported upon a somewhat enlarged firm base, 

 round which the tough basement-membrane of the "proboscis" is fixed. Below the line of 

 attachment of the latter the process is again narrowed, and presents just over the opening of the 



1 There would seem to be considerable variety in the structure of the body-wall of these forms. 

 Another species dredged in 125 fathoms off Cape Rosier, in the Gulf of St. Lawrence, by Mr. J. F. 

 Whiteaves, shows (in the spirit-preparation) underneath the glandular lining of the digestive chamber 

 all round a dense and almost cartilaginous layer marked on its inner surface by regular transverse striae, 

 but there is no specialization of chitinous tissue as in the Mediterranean examples. 



19 



