DEPARTMENT OF MARINE BIOLOGY. 209 



Thylacoplethns differs from other known Rhizocephala in that it does not 

 prevent the molting of the host. When this takes place the external sacs are 

 detached from the root system, but this will probably not prevent the larvae 

 from pursuing their normal development. The root system buds off a new set 

 of external sacs, so that the germ-cells are actually formed in the roots. These 

 cells grow considerably in size, and form a number of yolk globules, but the 

 fully formed egg is small for a rhizocephalan. The maturation processes 

 were not observed, but neither testis nor any spermatozoan-like bodies were 

 found, and there can be no doubt that reproduction is parthenogenetic. The 

 egg undergoes complete segmentation; the nauplius stage is passed through 

 within the egg membrane and eventually the larva hatches in the Cypris 

 stage. In the absence of a definite aperture to the exterior, it must be liberated 

 either by the molting of the host or the tearing of the external membrane. 



Thylacoplethus thus occupies a unique position among the Rhizocephala 

 on account of its simplicity of structure, a simplicity which is doubtless the 

 result of degsneration. The mantle cavity, nervous ganglion, testis, and repro- 

 ductive ducts which characterize the higher Rhizocephala are all lost here. 

 On the other hand, the development of a large number of external sacs from a 

 single root system is a new feature. In the complete segmentation of the egg, 

 Thylacoplethus differs from all other Crustacea. 



Commensal Animals Associated with Comatula. 



With practically every individual of Comatula parvicirra, a pair of alpheids, 

 male and female, is found, generally lying side by side on the disk of the 

 crinoid. The animals can move about quite freely, but are guarded against 

 detachment in the one species by the recurved double claws on the thoracic 

 legs and in the other by a great modification of the smaller chela, which is 

 usually fixed firmly in the flesh of the disk. The coloration of the alpheids 

 varies greatly and corresponds roughly with that of the host, which is also 

 very variable. The same variations occur in both species. 



A number of other forms are definitelj^ associated with Comatula as com- 

 mensals. These include a Munida, two kinds of shrimps, an amphipod, and 

 an isopod among Crustacea; various myzostomids and a polynoid polychsete; 

 and one or two species of ophiuroids. In most of these cases the coloration 

 shows some relation to that of the host. 



The Biology of the Gall-Forming Crab, Hapalocarginus. 



Hapalocarcmus was abundant at Murray Island in galls on the coral 

 Pocilloporo. Previously, the female only was known and the reproductive 

 habits were consequently a matter of conjecture only. It appears, however, 

 that the female alone forms galls, settling down to a sedentaiy existence when 

 still very small and immature. Only in the fully formed galls are the big, 

 mature females found with their swollen abdomen (occupied bj^ the large 

 ovar}^) and well-developed s\\ammierets. The mature male probably never 

 attains a size as great as even the smallest gall-inhabiting female, but is easily 

 recognizable by the copulatory stylets and large testes. It is probably free- 

 living, visiting the female when she becomes mature, entering the gall through 

 one of the respiratory apertures. The only male found was obtained in a 

 gall with a recently-molted female. It is perhaps misleading to apply the 

 term "gall" to the structures inhabited by Hapalocarcinus. The broadening 

 of the naturalh^ slender branches of the coral to form a protection for the crab 

 is due' to the respiratory currents of the crab, which merely controls the direc- 



