VISCERA. 131 



the mixiptervgium) from the fin proper; the liighly speciahzed form of the primar}' 

 skeletal parts — against the simpler form in the Plagiostomes (as the simple rod- 

 like shape of the terminal joint * * * ), " the presence of "particular copulatory 

 organs," and infers finally that "the Holocephales by no means occupy a primitive 

 position among the Selachians." (Danish Ingolf Exped., 11, pp. 20-21). 



VISCERA. 



In the development of its viscera, also, Chimaera indicates a high degree of 

 specialization. This, for example, may be noted in the following structures: 



Mesenteries. — No continuous mesentery is observed even in later embryonic 

 stages of development. Thus, in the embryo shown in plate vii, fig. 45, the 

 mesentery is clearly reduced to the string-like supports for vessels and ducts which 

 characterize the adult. In the same stage only a rudiment of a ventral mesentery 

 is present. 



Gtit. — In no stage is the gut of the same proportional length as in the shark. 

 In the latter (Pristiurus) the length of the digestive tract (measured from mouth to 

 anus) decreases in length between stages k to o from 55 per cent to about 50 per 

 cent of the total length of the embryo; in Chimsera in similar stages from less than 

 30 to about 15 per cent. In other words, the gut of Chimaera develops in a much 

 more restricted body region; and from early stages it appears as a short tube of 

 wide caliber. The stomach dilatation, we may thus conclude, fails to become 

 expressed, and the intestinal valve, instead of undergoing the further spiral devel- 

 opment of sharks, makes but a few turns (about four) and then increases rapidly 

 in the width of the infolded band. 



Gills. — The gills exhibit greater changes in their "metameral" series than 

 sharks. Thus the hyobranchial cleft, even in as early a stage as k, is notably the 

 largest and by stage o the opercular fold has attained almost its adult proportion. 

 On the other hand, the fifth gill-furrow, although clearly indicated, e. g., stages 

 K, L, M, fails to become a functional gill-slit. And the spiracle, even in a favorable 

 stage, is little more than a tubular rudiment; it never develops respiratory filaments 

 and is lost by stage n. Another evidence of precocious development is shown in the 

 mode of growth of the external gills. These filaments are from the beginning 

 (about stage k) of large caliber {cf. Schauinsland's Taf. xiv, fig. iio), /. c, they at 

 once assume nearly their functional size. Accordingly they do not arise in a 

 uniformly developed vertical series, but on account of their extraordinary diameter 

 bud out one after another as the gill-bar increases in size. Their later specialization 

 in developing blood-producing dilatations has already been noted (pp. 60, 106). 



Kidney. — The restricted length of the visceral cavity is accompanied by modi- 

 fications of the excretory system. Of the pronephros I am unable at present to 

 give a detailed account, and will note only that it is smaller and more difficult to 

 trace than in the shark. The mesonephric tubules, on the other hand, are long and 

 coiled irregularly ; they appear early and are clustered in a deep stroma along 

 the dorsal wall of the body cavity. Their early condition, therefore, does not, 



