THE ISOPODA 



Sense-Organs. — In Munna and some allied Asellota the eyes are 

 set on prominent lateral lobes of the head, but there is no evidence 

 that these represent the movable ocular peduncles of the primitive 

 Malacostraca. The number of oramatidia in each eye varies from 

 four in Asellns to about 3000 in Bafhi/nomus. The crystalline body 

 is generally bipartite, but in one of the ommatidia in each eye of 

 Asellns it is tripartite. The number of retinular cells and of 

 rhabdomeres may be 4, 5, or 7 in ditferent genera. 



The only Isopoda in which statocysts have been observed are 

 Anthura gracilis,'^ and another species of Anthiu'idae, where they 

 have recently been described by Thienemann. A pair of them are 

 situated in the anterior part of the telson. Each communicates 

 ^\'^th the exterior by a fine canal and contains a single statolith. 

 ^Muscles are attached to the wall of each statocyst. In view of the 

 sluggish movements and burrowing habits of the Anthuridae, their 

 possession of these organs is somewhat remarkable. 



Reproductive Sijstem. — The ovaries vary in form, but are generally 

 elongate and tubular and are not connected with each other in 

 the middle line. In some Epicaridea they give off segmentally 

 arranged diverticula. The oviducts are short and simple, occasion- 

 ally (Aselhs) dilating to form a sperm receptacle. A peculiarity 

 which is quite unique among Crustacea is presented by certain 

 Epicaridea (Hemioniscidae, Liriopsidae) which have two pairs of 

 oviducts. As the oviducts, or at least their external apertures, are 

 not developed until the segmentation of the body has disappeared 

 in the adult females, it is not possible to determine Avhether both 

 pairs belong to one somite. 



A remarkable cycle of changes takes place in the female repro- 

 ductive organs of the Oniscoidea. When sexual maturity is reached, 

 but before the oostegites have developed, the generative apertures 

 are present in the usual position, but instead of communicating 

 with the oviducts, each leads into a blind invagination of the integu- 

 ment, which functions as a receptaculum seminis. After this has 

 been filled with sperm in copulation, it acquires a communication 

 with the oviduct and the sperms pass up into the ovary. At the 

 next ecdysis the receptacula disappear and the oviducts no longer 

 communicate with the exterior. The fertilised eggs are stated to 

 pass into the body-cavity and from thence to the marsupium by 

 Avay of a slit-like impaired aperture between the last two thoracic 

 somites. This statement, however, can hardly be accepted without 

 further confirmation, as the existence of a free opening from the 

 body-cavity (haemocoel) to the exterior would be quite unparalleled 

 among Arthropoda. A second lot of eggs are fertilised by sperms 

 remaining in the oviducts and pass into the marsupium after the 

 first brood have left it. After Ulceration of the second brood, 



^ According to Gurney, tlie .sjiecies is really Cyathura carinata (Krciyer). 



