210 THE CRUSTACEA 



continued into a posterior median aorta. In place of this, however, 

 a pair of abdominal arteries (which may unite into one immediately 

 after their origin), springing from the ventral surface of the heart 

 (or, exceptionally, from the last pair of thoracic arteries), run 

 backwards and send off numerous branches. In certain forms with 

 natatory pleopods (Conilera) where the muscles of the abdomen are 

 greatly developed, these vascular ramifications attain a remarkable 

 degree of complexity. The minute subdivision of the ultimate 

 arterial branches is also well shown by the vessels supplying the 

 hepatic caeca. 



From the lacunae of the haemocoel the blood is carried to the 

 branchial pleopods by sinuses which vary somewhat in their 

 arrangement in the different types. A median ventral abdominal 

 sinus is nearly always present from which afferent branchial vessels 

 are given off. Sometimes there are also two great sinuses running 

 along the lateral margins of the thorax. From the pleopods the 

 blood is returned by efferent branchial vessels to the pericardium. 

 In addition to the arterial blood thus received, it appears that a 

 small amount of venous blood may also enter the pericardium by 

 some small apertures in its anterior part communicating with the 

 general lacunar system of the body. The existence of these apertures 

 is important as a starting-point for comparison with the very 

 different circulatory system of the Amphipoda. 



Excretory System. The antennal gland of the Isopoda, unlike 

 that of the Amphipoda, appears to persist only in a vestigial 

 condition. In Asellus and some Oniscoidea it has been recognised 

 as a small vesicle or a solid mass of cells without communication 

 with the exterior. A well-developed maxillary gland of the usual 

 type has been found in Asellus, and in Ligidium and some other 

 Oniscoidea. In the Oniscidae it is reduced in size, and appears in 

 some cases to have no external opening. It has been suggested that 

 in some of the terrestrial species it may have a salivary function. 



It seems probable that in many Isopoda the excretory products 

 are got rid of by being stored in the so-called " fat-bodies." An 

 excretory function has also been attributed to certain glands opening 

 on the ventral surface of the posterior thoracic and abdominal 

 somites. 



Nervous System. The ventral nerve-chain presents various 

 degrees of concentration and coalescence of the ganglia, not always 

 in correspondence with the degree of fusion of the somites. In 

 Chiridotea and Sphaeroma seventeen distinct ganglia are found, 

 corresponding to all the post -oral appendages, and in Sphaeroma 

 an additional ganglion is found in the telson which is not repre- 

 sented in any other Eumalacostracan. In most cases, however, the 

 ganglia in front of the second thoracic form a single mass, and not 

 more than four ganglia are generally distinct in the abdomen. 



