396 PEOF. a. B. HOWES OH THK 



is concerned, there may be derived that met with in all other 

 living Plagiostomi. 



It remains now to consider, more closely than heretofore, the 

 differences and resemblances between the appendix digitiformis 

 and its "duct" and those structures to which the terms caecum 

 coli and appendix vermif ormis have been applied. The appendix 

 digitiformis lies to the left side of the valved intestine, as pointed 

 out by Blanchard (2. p. 190) * and its apex is, in the adult, 

 usually directed towards the animal's right. Examination of it 

 in relation to the lumen of the gut shows it to be, however, a 

 derivative of the mid-dorsal intestinal wall. On opening the 

 body-cavity from the ventral aspect in the Common Skate, the 

 structure in question is seen to occupy the interspace between 

 the pyloric sac of the stomach and the adjacent intestine, and to 

 be disposed lineally with the former. Anteriorly it is received 

 into a notch in the posterior border of the pancreas. Such may 

 be the disposition of the caecum among certain of the higher 

 Vertebrata, and in Rana and Lacerta that organ, if examined 

 with sufficient care, may generally be found to lie at or towards 

 the side t. 



The appendix digitiformis audits " duct " are, like the appendix 

 vermiformis and caecum coli, extremely variable in development 

 in even allied forms %. Figs. 8 to 14 represent the former in 



" pseudobraiich " as recently defined by Sagetnahl (Morphol. Jabrb. t. x. 1885, 

 p. 113), has just been shown by Virchow (Vei-handl. d. physiol. Gresellsch. 

 Berlin, 1889-90 [Archiv. f. Anat. u. Phys., Phys. Abth. 1890, pp. 170 and 178 

 et segg'.]) to differ from that of all other Chondrichthyes in having the essential 

 structure and the capillary networks of a true demibranch ; and it is difficult 

 to believe, in view of his researches, that it can be other than respiratory in 

 • function. 



* In Lcemargus this is so markedly the case that the suspensory mesentery 

 of the gland shifts its position, and becomes attached obliquely across the 

 antero-dorsal moiety of the same. 



1" This fact would appear to account for the representation of the same as a 

 diverticulum of the ventral wall of the gut (cf. Eana [Marshall, 22. p. 27, 

 fig. 5] and Amphisbana [Bedriaga, 1. pi. iv. fig. 2]). 



\ Blanchard has directed attention to this (2. p. 182) so far as the appendix 

 digitiformis is concerned. He gives as the minimum length observed 08 centim. 

 (^Chiloscyllmm -plagiosum, length of body not stated). In a C. ocellatmn of 

 46 centim., recently captured by my friend Prof. A. 0. Hadden in the Torres 

 Straits, and by him generously placed at my disposal, the gland measures 

 '0-6 centim. - 



