268 



COMPARATIVE ANATOMY. 



of this pharynx projects into the mid-gut. The walls of this portion 

 are, as a rule, stronger, and its inner surface is often distinguished 

 by the possession of a firm chitinous framework, which is pro- 

 vided with tooth-like processes turned towards one another, and 

 moved by muscles ; these ridges, spicules, or seta?, are derived 

 from the chitinous membrane, which invests this portion. They 

 form an apparatus which serves for the comminution of the 

 ingesta ; and hence this portion is known as the masticatory 

 stomach. As a rule it is of considerable size, and is regular in form, 

 owing to its firm framework. It is most largely developed in the 

 Decapoda (Fig. 143, v). In the Eutomostraca it is small, or not 

 developed at all, while in the Isopoda among the Arthrostraca the 

 small masticatory stomach is provided with a fairly complex frame- 

 work, of which there are indications also in the Amphipoda 

 (Gammarus). 



The mid-gut (Fig. 130 I) forms by far the longest portion of the 



**■< 



Fig. 13G. Organisation of a Daphnia. a Tactile antennas, gs Cerebrum, oc Eye 

 i Enteric canal (mid-gut). h Csccal tubes at its commencement, g Shell gland, 

 c Heart. I Labrum. ov Ovary, o An Egg in the brood-space (o') formed between 



the body and the mantle (after Leydig). 



enteric tube, and varies greatly in width, and in the formation of 

 cascal diverticula. In many cases it has the same calibre all along; 

 in others it is widened somewhat anteriorly, or mesially ("Chyle- 

 stomach"); or the widened portion extends over the whole of the 

 mid-gut ("Chyle-intestine" of the Isopoda). 



There are caecal diverticula at the commencement of the mid- 

 gut, in all orders of the Crustacea. They arise as paired, and 

 seldom as impaired, caeca. In the Copepoda they are found 

 only in a few genera; in the Branchiopoda they are more common, 

 and form either a single pair of short csecal tubes (Fig. 136, h) 

 (Daphnida), or are more richly branched (Argulus, Hedessa), or 

 arise from the enteron in greater numbers, and are differentiated 

 at their ends into glandular organs (Apus). We find that the same 

 phasnomenon of metamorphosis of the enteric cajca into secreting 

 organs obtains in the Malacostraca, and at exactly the same spot. 



