The Anatomy of Chlamydoselachus 415 



teries of the organs in this region, and it penetrates the substance of the spleen by fol' 

 lowing the blood vessels which supply this organ." 



I was unable to find, by dissection, any undoubted pancreatic ducts. Such ducts 

 are presumably present, unless they have disintegrated through long preservation of the 

 material. Hawkes (1907) does not mention any pancreas in Chlamydoselachus, but 

 describes a pancreatic duct which probably belongs to the dorsal pancreas since it opens 

 into the valvular intestine where the spiral valve begins. 



Collett's (1897) description of the pancreas in his specimen of Chlamydoselachus is 

 interesting in that he speaks of dark and light portions of the pancreas. His description 

 is quoted in full: 



The pancreas consists of two large lobes, of which each is subdivided into an upper 

 and lower portion, so that it really is in four divisions, of which the two hinder portions are 

 lighter in color than the front ones. On the right side it forms, first, a short light-colored 

 lobe, about 80 mm. long and 35 mm. broad. Anteriorly, it is almost entirely separated from 

 a curved front portion, which is of darker hue than the hinder part. Posteriorly there also 

 exists a lower portion, of a length of about 100 mm. ; above this hes a darker-colored portion 

 whose length is about 48 mm., which adjoins the hinder hghter part, and is connected with it. 



Although CoUett does not mention a spleen, it seems likely that the dark organs 

 described by him are accessory spleens. 



Deinega's (1925) drawing (his Fig. 1) of the digestive system of Chlamydoselachus 

 does not show any organ labeled pancreas, but his Fig. 2 is a drawing of a section of some 

 tissue said to have been taken near the pancreas. In it he distinguishes blood vessels, 

 fibers and cells. He suggests that it may be splenic tissue. Evidently this material was 

 in a very poor state of preservation for microscopical study. 



In Heptanchus (Daniel, 1934, Fig. 119) both dorsal and ventral pancreases are present 

 and well developed. Their relations, as shown in this figure, appear to be much the same 

 as in Chlamydoselachus. In another figure by Daniel (1934, Fig. 120) the names of the 

 two divisions of the pancreas appear to have been interchanged. 



ORGANS ASSOCIATED WITH THE DIGESTIVE TRACT 



For convenience there are included in this section brief descriptions of two organs 

 that are topographically related to the digestive system, but are not a part of it: the 

 thyroid gland, which develops from the distal portion of a diverticulum from the floor 

 of the pharynx; and the spleen, which has no developmental relation to any part of 

 he digestive system. 



THE THYROID GLAND 



The position of the thyroid, attached to the ventral surface of the basihyoid cartilage, 

 is shown in my Text'figure 26a, p. 361, after Goodey, 1910.1; also by Goodey (1910.2) 

 in his Fig. 1 ; and by Allis (1923) in his Fig. 38, pi. XIV. 



The thyroid of Chlamydoselachus is especially interesting because, in the adult, 

 it sometimes retains a primitive or embryonic feature. Phylogenetically, the thyroid is 



