NO. 10 CRUSTACEAN METAMORPHOSES — SNODGRASS 5 



lead to, but actually parasites are highly specialized for the life they 

 lead by a simplification of structure and the elimination of all un- 

 necessary organs, which were indispensable to their free-living an- 

 cestors. In fact, no s)^mpathy need be wasted on "degenerate" para- 

 sites ; give them credit for having found a simple and easy way of 

 living, though at the expense of another creature. They have discarded 

 all useless equipment, and some of them have devised most ingenious 

 ways of attacking the host. 



The control of metamorphosis by hormones has been extensively 

 studied in insects, but apparently no comparable studies have been 

 made on the role of hormones in the metamorphosis of crustaceans. 

 It is well known that hormones are produced in the eyestalks of 

 decapods, and the source of the eyestalk hormones has usually been 

 referred to two organs known as the sinus gland and the X organ. 

 However, from recent investigations (see Bliss and Welsh, 1952; 

 Passano, 1953) it is now known that the so-called sinus "gland" is 

 not a gland but a complex of the enlarged ends of nerve fibers pro- 

 ceeding from the X organ and from numerous neurosecretory cells 

 in the brain, in the ganglia of the optic lobe, and in the thoracic 

 ganglia. The sinus "gland" is therefore a receiving and distributing 

 center for various hormones. Functions that have been attributed to 

 these hormones include the movement of pigment in the compound 

 eye, regulation of chromatophore activity in the integument, control 

 of moulting, and the rate of development of the ovaries. Knowles 

 (1953) gives evidence that the chromatophores are activated also by 

 neurosecretory cells in the region of the tritocerebral commissure and 

 the postcommissural nerves. The control of moulting by lengthening 

 the period between moults was attributed by Passano to the X organ, 

 which is itself a neurosecretory tissue in the proximal ventral part of 

 the medulla terminalis of the optic lobe. Removal of both sinus 

 "glands" has no effect on moulting since the hormone can escape from 

 the cut ends of the nerves. Panouse (1946) also, in a study of 

 Leander, had claimed that the "sinus gland" produces a hormone that 

 normally blocks the growth of tissues and thus causes a lengthening 

 of the intermoult period and retards the maturing of the ovaries. 



From later work by Gabe (1953) and Echalier (1954), however, 

 it now appears that moulting, at least in the Malacostraca, is controlled 

 by a pair of ductless glands in the antenno-maxillary region. These 

 glands, discovered by Gabe, are named by him the Y organs, and were 

 demonstrated to be present in 58 malacostracan species, ranging from 

 Nebalia to the decapods and stomatopods. In species in which the 

 excretory gland is maxillary, the Y organs are in the antennal seg- 



