INTESTIBTAI; CANAL OF THE ICHTHTOPSIDA. 393 



rectum, when differentiated, with the cloaca). The posterior 

 series appear to be wanting only when (espec. Teleostei, Mar- 

 supialia) the anterior splanchnic ones supply the entire viscus. 

 Whether this condition results from the suppression or absence 

 of the posterior series, or from anastomosis between it and the 

 anterior, there is, at present, no evidence to show. I have 

 examined a large number of Teleostei, in vain, in the hope ot 

 finding, in that order, a more general development of the inferior 

 mesenteric trunks, as defined by myself. 



III. On certain Appendages of the Intestinal Wall of the 

 Ichthyopsida, in relation to their Arterial Supply. 



If the foregoing considerations are sound, it will follow that 

 the so-called inferior mesenteric artery of the Plagiostomes 

 (a.sm., fig. 1) really represents that vessel (the posterior vessel of 

 the anterior splanchnic series) which is normally distributed to 

 the head of the large intestine with its adjacent parts, in the 

 higher Yertebrata. If this be so, that which we, in the Piagio- 

 stome fishes, customarily regard as the rectum, would appear to 

 represent the entire large intestine together with a portion of 

 the small one, as defined for the terrestrial Vertebrata. 



The most conspicuous structure present in this region of the 

 Plagiostome's gut is the processus digitiformis^ {dv", fig. 1). On 

 comparison with the Frog f (fig. 4) or with a Lizard J, this struc- 



* Better known as the "rectal gland" ("bursa cloacae" of Eetzius). I 

 prefer to exclude these terms from the text, for reasons which the sequel will 

 show. 



t That the Frog possesses a rudimentary cacum is at once clear on com- 

 parison with a Lizard. If the head of the large intestine be opened from the 

 side, as represented in fig. 3, it will be seen that the ileum enters that viscus 

 from below ; a line drawn vertically through the valvular extremity of the 

 ileum will pass behind an overhanging enlargement of the antero-dorsal wall 

 of the large intestine, and that this represents the ccecum coli is clear on con- 

 sideration of the well-known facts of morphology of that structure. 



The discovery of the Frog's cajcum has been attributed to myself, in error 

 (Wiedersheim [33], p. 564). I was the first to figure it (' Atlas of Practic. 

 Elem. Biology,' 1885, pi. 1. fig. 13) ; it was originally described by Huxley in 

 Huxley and Martin's ' Elem. Biology,' original ed. p. 166 (1875). 



+ Cf. Parker, ' Zootomy,' p. 164 and fig. 40. 



