228 Koyal Society, 



readily be examined microscopically with high powers, and it is then 

 easily observable that its fibrous investment is a completely shut 

 sac. In Rhynchonella the enlarged caecum is often full of diatoma- 

 ceous shells, but it is impossible to force them out at its end, while 

 if any aperture existed they would of course be readily so extruded. 



However anomalous, physiologically, then, this caecal termination 

 of the intestine in a molluscous genus may be, I see no way of 

 escaping from the conclusion that in the Terebratulidce (at any rate 

 in these two species) it really obtains. There are other peculiarities 

 about the arrangement of the alimentary canal, however, of which I 

 can find either no account at all or a very imperfect notice. 



The intestinal canal (figs. 1 and 2 h, d, e) has an inner, epithelial, 

 and an outer fibrous coat ; the latter expands in the middle line into 

 a sort of mesentery, which extends from the anterior face of the 

 intestine between the adductors, to the anterior wall of the visceral 

 chamber, and from the upper face of the intestine to the roof of the 

 visceral chamber ; while posteriorly it extends beyond the intestine as 

 a more or less extensive free edge. I will call this the mesentery (/). 



From each side of the intestinal canal, again, the fibrous coat gives 

 off two ' bands,' an upper (^), which stretches from the parietes of the 

 stomach to the upper part of the walls of the visceral chamber, 

 forming a sort of little sheath for the base of the posterior division 

 of the adductor muscle, which 1 will call the gastro-parietal band ; 

 and a lower, which passes from the middle of the intestine to the 

 parietes, supporting the so-called ' auricle.' I will call this the ilio- 

 parietal band {h). 



The ilio-parietal and gastro-parietal bands are united by certain 

 other ridges upon the fibrous coat of the intestine, from whose point 

 of union in the middle line of the stomach posteriorly, a pyriform 

 vesicle (w) depends. 



The mesentery divides the liver into two lateral lobes, while the 

 gastro-parietal bands give rise to the appearance that these are again 

 divided into two lobules, one above the other. I am inclined to think 

 that these bands are what have been described as * hepatic arteries,' 

 at least there is nothing else that could possibly be confounded with 

 an arterial ramification upon the liver. 



This description applies more especially to Rhynchonella and 

 Waldheimia, but the arrangement in Lingula is not essentially dif- 

 ferent. 



2. The Circulatory System of TerebraiulidcB. — Considerable differ- 

 ences of opinion have prevailed among comparative anatomists as to 

 the nature and arrangement of the vascular system in the Brachio- 

 I)oda. A pair of organs, one on each side of the body, have been re- 

 cognized as Hearts since the time of Cuvier, who declared these 

 hearts in Lingula to be aortic, receiving the blood from the mantle 

 and pouring it into the body, the principal arterial trunks being dis- 

 tributed into that glandular mass which Cuvier called ovary, but 

 which is now known to be the genital gland of either sex. 



Professor Owen in his first memoir follows Cuvier's interpretation, 

 stating that in Orbicula the pallia 1 veins terminate in the hearts. 



