90 Papers from the Department of Marine Biology. 



third much smaller; flagellum composed of about 8 joints. Second antenna 

 more slender than the first but not very much longer; in the peduncle the first 

 two joints are very small and equal; the third is as long as the first two 

 together, the fourth longer still, and the fifth about as long as the third; the 

 flagellum is composed of 7 joints and extends very little beyond the lateral 

 margin of the first segment. The frontal lamina is pentagonal, scarcely twice 

 as long as broad. 



Maxilliped second joint much longer than those succeeding, which are very 

 broad. First thoracic segment nearly twice as long as the second; the pos- 

 terior six segments vary slightly among themselves. Appendages robust, all 

 joints broad, with few setse. 



First segment of abdomen quite hidden below last thoracic. Telson forms 

 an equilateral triangle, ending in a sharp apex with 16 to 18 spines on the pos- 

 terior margin, surface smooth. Inner ramus of uropod extends a little further 

 than the telson, is narrow and ends in a sharp apex; both margins are fur- 

 nished with spines; outer ramus about the same length, but still narrower; 

 peduncle not produced far posteriorly on inside. 



Appendix masculina slender, much longer than inner ramus of second uro- 

 pod, apex rounded. 



This species is very distinct. It is separated from all others by the 

 insignificant length of the second antenna, and also, so far as I can find 

 from the published descriptions, by the character of the colour mark- 

 ings. The shape of the telson and especially of the uropods is also very 

 distinctive. 



All the members of the Cirolaninse to which this species belongs 

 are predatory and swim about very actively. Cirolana lineata is far 

 from being as sedentary as Synalpheus, which, while very energetic at 

 times, rests for long periods on the disc of the crinoid. While the isopod 

 may make busy excursions onto the surface of Comanthus, it is often to 

 be seen diving into the gut of the host, where it apparently spends a 

 large part of its time. It does not feed, so far as I know, on the tissues 

 of the crinoid itself, but only on the food it finds in the stomach. 



It must be this or a very closely related form which Haswell refers to 

 as collected from a crinoid in Torres Straits associated with Synalpheus 

 comatularum and Galathea deflexipous, but no identification of his isopod 

 seems to have been made. 



This case of commensalism is interesting because of the faint but 

 significant response to the colour stimuli of the crinoid. In nearly all 

 the species of this genus, where the colouration is noted at all, the chro- 

 matophores appear to be distributed over the surface so as to form a 

 series of evenly spaced dots or dashes of pigment. The concentration 

 of these into lines and the development of a purplish pigment seem 

 to be the first effects of the commensalism. The insignificant quanti- 

 tative development of the pigment is probably associated with the very 

 active life of the isopod and its frequent immersion in the gut of the 

 host, rather than to any incapacity of the isopod to assume the colours 

 of the environment. 



