ANNULOSA: CRUSTACEA. 197 



of the mouth is situated between the bases of the mandibles, 

 bounded behind by a forked process, called the " labium," or 

 " metastoma," and in front by a single plate, called the " la- 

 brum" (upper lip). The next segment bears the long antennae, 

 or feelers (fig. 71, ga) t each consisting of a short protopodite, 

 and a long, jointed, and segmented endopodite, with a very 

 rudimentary exopodite. In front of the great antennae are the 

 next pair of appendages, termed the " antennules," or smaller 

 antennae (fig. 71, a), each composed of a protopodite, and a 

 segmented endopodite and exopodite, which are nearly 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 prolonga- 

 tion 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 cir- 

 culation 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., Copepodd) 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 inter- 

 stitial lacunar system arise the venous trunks, which are gene- 

 rally 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 en- 

 tirely, in driving the aerated blood to all parts of the body. 



Distinct respiratory and circulatory organs may be altogether 



