ANNULOSA : CRUSTACEA. 225 



of equal size. Finally, attached to the first segment of the 

 head are the eyes, each of which is borne upon an eye-stalk 

 formed by the protopodite. The gill-chamber is formed by a 

 great prolongation downwards of the pleura of the thoracic 

 segments, and the gills are attached to the bases of the legs. 



As regards the digestive system of the Crustacea, the ali- 

 mentary canal is, with few exceptions, continued straight from 

 the mouth to the aperture of the anus. There are no salivary 

 glands, but a large and well-developed liver is usually present. 

 A heart is generally, but not always, present. In most of the 

 lower forms it is a long vasiform tube, very like the " dorsal 

 vessel" of Insects. The exact course of the circulation has 

 been differently stated by different writers, but the following 

 appear to be the facts of the case : In some of the lower forms 

 (e.g., Copepoda) there are no arterial vessels, and the venous 

 blood returned from the body is collected into a venous sinus 

 the so-called " pericardium," which surrounds the heart and 

 opens into it by valvular apertures. In the higher forms, the 

 heart gives off a number of arteries by which the blood is 

 driven to all parts of the body and to the gills. The arteries 

 do not terminate in a system of capillary Vessels, but in a 

 series of irregular lacunae occupying all the interstices between 

 the different organs of the body. From this interstitial lacunar 

 system arise the venous trunks, which are generally dilated 

 into more or less extensive sinuses. Whether the whole of 

 the venous blood is submitted to the action of the gills, or 

 whether the blood sent to the gills is derived mainly from the 

 heart, is a matter of question ; but the former is the more 

 probable view. Be this as it may, the blood is invariably 

 returned to a large venous sinus which surrounds the heart, 

 and opens into it by a number of valvular apertures. It 

 follows from this description, that the heart of the Crustacea 

 is mainly, if not altogether, a systemic heart, being concerned 

 chiefly, if not entirely, in driving the aerated blood to all parts 

 of the body. 



Distinct respiratory and circulatory organs may be altogether 

 wanting ; but, as a rule, distinct branchiae are present. The 

 exact form and structure of the gills differ in different cases, 

 but their leading modifications will be alluded to in treating of 

 the different orders. 



TABULAR VIEW OF THE DIVISIONS OF THE CRUSTACEA, 

 Sub-class I. EPIZOA (Haustellata\ 

 Order i. Ichthyophthira. 

 2. Rhizocephala. 



P 



