PHYSIOLOGY OF THE INVERTEBRATA. 75 



retractile tentacula without suckers ; while the Dibranchiata 

 possess two gills and from eight to ten tentacula with suckers 

 round the head. 



From what has been said, it will be seen that the Mollusca 

 have a very complete digestive system which is comparable, 

 in a great measure, with that of the Vertehrata. In some 

 of the Mollusca we find a long oesophagus and enlarged 

 stomach, an intestine with circumvolutions, and a rectum. 

 In others, we observe the stomach arranged more or less 

 after the plan of certain Vertebrates ; there are cardiac and 

 pyloric portions, separated by a salient fold. " Sometimes 

 the stomach is furnished with triturating hooklets of varied 

 form. But it is especially by the development of the 

 glandular appendages that the stomach of the Mollusca is 

 distinguished from that of the animals hierarchically inferior. 

 These organs, in fact, go on perfecting and complicating 

 themselves more and more in the diverse families of Molluscs, 

 and especially in the more advanced of the Molluscs — the 

 Cephalopods (Cuvier) — ^there are oesophageal salivary glands 

 with short caeca, a liver (so-called) developed, compact, 

 divided into lobes, provided each of them with an excretory 

 conduit or duct, and all these conduits or ducts open together 

 or separately at the commencement of the median intestine, 

 or into the stomach." But this " liver " is essentially pan- 

 creatic in function : the true Vertebrate liver is entirely 

 absent in the Invertebrata. 



The Phaeyngopneustal Series. 



This series is divided into two orders — the ITemichordata 

 and the Vrochordata (Tunicata). 



(l) The Hemichordata are represented by a single example 

 — Balanofflossus, which is " an elongated, apodal, soft-bodied 

 worm, with the mouth at one end of the body and the anus 

 at the other." The mouth, surrounded by a well-marked 

 lip, leads into a wide oesophagus which opens into a stomach. 



