THE TANAIDACEA 193 



female. The iiropods are nearly terminal on the last segment of 

 the body. In Apseudcs the two rami are long, multiarticulate, 

 and tlagelliform. In other cases the rami may be much reduced 

 and the exopodite is sometimes wanting. 



Internal Anatomy. — In Ajneudes, the alimentary canal has three 

 pairs of hepatic caeca. The heart is elongated, extending through 

 the six free somites of the thorax, giving off a median aortic vessel 

 anteriorly and a pair of diverging abdominal arteries (but no 

 median vessel) posteriorly. In Leptochelia and in the young of 

 Apseudes it has two pairs of ostia, but in the adult Apseudcs the 

 right anterior ostium disappears and those of the posterior pair 

 become asymmetrically placed. In Tanais only one pair persists. 

 The anterior aorta, after dividing to encircle the brain in the 

 median j^h^ne, forms a circumoesophageal ring, but there is no 

 subneural sternal arter}'. The lateral folds of the carapace are 

 traversed by a network of blood-channels supplied by branches 

 from the anterior aorta, and no doubt form the chief organs of 

 respiration, possibly assisted by the epipodites of the maxillipeds. 



Considerable importance has been attached to the thoracic 

 position of the heart in the Tanaidacea as differentiating them 

 from the Isopoda and indicating affinity with the Amphipoda. As 

 a matter of fact, however, in certain Isoj^oda (Jaera) the anterior 

 end of the heart extends as far forward as it does in Apseudes, 

 and the suppression of the abdominal portion, leaving intact the 

 paired abdominal arteries, would produce a disposition of parts 

 essentially similar to that of the Tanaidacea. 



The maxiUarij (jland is well developed in Apseudes, and a vestige 

 of the antennal gland has been descril)ed. Dermal (jlands are 

 commonly found on the body and limbs, and in some Tanaidae the 

 secretion appears to be utilised in forming the tubes of mud in 

 which the animals live. In Ileterofanais groups of gland-cells are 

 described situated on each side of the anterior thoracic somites and 

 opening by long ducts on the terminal segments of the correspond- 

 ing legs, an arrangement which recalls that found in certain 

 Amphipoda. The nerve-chain in Apseudes has all the ganglia of the 

 post-oral somites distinct. 



It was stated by F. Miiller that a species of Tanais possessed an 

 open statocyst-cavity containing a statolith in the basal segment of 

 the antenuule, l)ut the observation has not been confirmed. 



The reproductive organs of both sexes are of a simple type. 

 The vasa deferentia, in Apseiides, open close together on a median 

 process of the last thoracic sternum. A seminal vesicle is formed 

 in Tanais and in Leptochelia by fusion of the two vasa deferentia, 

 but here also the external opening is paired. 



Sexual differences are often strongly marked. The olfactory 

 filaments of antennules and antennae are, as usual, more numerous 



13 



