80 PL ATT. [Vol. V. 



I have not attempted a complete account of any of the sys- 

 tems of the head, thinking it sufficient to note those points in 

 which I have found Acanthias to differ from Selachians hitherto 

 described. 



A. Head Cavities and Muscles. 



The origin and the relations of the first four or five head 

 cavities in Acanthias are in several respects exceptional. To 

 make these differences clear, I shall refer briefly to the appear- 

 ance of these cavities in other Selachians, as described by Bal- 

 four, Marshall, Van Wijhe, and Dohrn. 



In regard to the head cavities in Scyllium, Marshall (No. 10) 

 says that by stage D the " separation of somatic and splanchnic 

 layers . . . has gone so far as to give rise to a distinct cav- 

 ity in the head region on either side, while at a considerably 

 later period the separation extends further back, so as to form 

 in the trunk the peritoneal or body cavity. The whole of the 

 cavity so formed may be termed ccelom." This cavity is then 

 divided into anterior and posterior parts by the appearance of 

 the hyomandibular clefts. In regard to succeeding stages, Mar- 

 shall quotes from Balfour (No. i) as follows: "During stage 

 I, this front section of the head cavity (the part in front of the 

 hyomandibular cleft) grows forward and becomes divided, with- 

 out the intervention of a visceral cleft, into an anterior and a 

 posterior division. . . . Between stages I and K, the anterior 

 (premandibular) cavities of the two sides are prolonged ven- 

 tral wards, and meet below the base of the fore brain." There- 

 fore, according to Marshall, the premandibular cavity in Scyllium 

 is cut off from the anterior part of the original ccelomic cavity. 

 This is not the case in Acanthias. Here, the premandibular 

 cavity arises independently in the wide mass of cells in which, 

 at an early stage, the anterior end of the notochord is lost. 

 This mass of cells gives rise to two pairs of cavities laterally 

 situated, and to two or three or even more median cavities. 

 Of the paired cavities, the more lateral and anterior are as yet 

 unnamed ; the more median and posterior form the lateral por- 

 tions of the future premandibular cavity. Both pairs of cavities 

 have definite walls and are constant in their appearance. The 

 median cavities, on the other hand, have irregular outlines, and 

 their number varies, unless I mistake, in different embryos at 



