234 THE CRUSTACEA 



attributed, mainly on the ground that they sometimes contain 

 calcareous concretions ; as they are outgrowths of the mesenteron, 

 it is impossible to regard them as homologous with the Malpighian 

 vessels of insects. In the Hyperiidea the anterior part of the 

 mid-gut may be dilated while the posterior part remains narrow. 

 It is stated to be lined by a very delicate cuticle of non-chitinous 

 nature. The proctodaeum is short, rarely reaching forward to 

 the posterior limit of the thorax, or, in the Caprellidea, into the 

 penultimate thoracic somite. An " anal gland " in the form of a 

 small diverticulum is described in Sijnurella. 



Circulatory System. — The circulatory system of Amphipoda is 

 remarkable for the imperfect development of the arterial vessels 

 and the consequent lacunar character of the greater part of the 

 circulation ; as compared watli that of the Isopoda it differs also in 

 the position of the tubular heart, which extends through the greater 

 part of the thoracic somites but does not reach the abdomen. It 

 lies in a spacious pericardium which may extend backwards into 

 the abdomen. Three pairs of ostia are present in most Gammaridea, 

 but Corophium has only one pair ; the Caprellidea have three pairs ; 

 in the Hyperiidea only two pairs are present as a rule, but in 

 Fhronima, although the male has only two, the female has three 

 pairs. The heart is continued at either end into the anterior and 

 posterior aortae, which are defined by valves. In addition, a pair 

 of arteries supplying the eyes and adjacent parts may arise from 

 its anterior end, but only in the Hyperiidea are there two or three 

 pairs of lateral arteries arising from the heart in the thorax and 

 comparable to the thoracic arteries of the Isopoda. The anterior 

 aorta is stated to divide in the median plane to encircle the brain 

 in a " pericerebral vascular ring '" very characteristic of the order, 

 with which is connected (except in Caprellidae) an oesophageal 

 ring similar to that of the Isopoda. It seems probable, however, 

 that these rings are rather lacunar spaces than well-defined vessels. 

 There is no trace of the subneural artery commonly developed in 

 Isopods, the oesophageal ring opening posteriorly into the great 

 ventral sinus which extends through the whole length of the body, 

 and into which the posterior aorta also empties its contents at 

 or near the end of the abdomen. Except perhaps in those forms 

 (Hyperiidea) which possess lateral thoracic arteries, the appendages 

 of the thorax and abdomen receive their blood-supply from this 

 ventral sinus. Within the appendages the afferent blood-streams 

 are contained in well-marked vessels which send branches to the 

 gills. The efferent vessels of the appendages unite to form in each 

 somite (except sometimes in the abdomen) a pair of pericardial 

 vessels by which the blood is returned to the heart. 



In comparing the circulatory system Avith that of the Isopods, 

 it is clear that the course followed by the blood in the abdominal 



