On the Rhizocephalan Genus Thompsonia, etc. 7 



PRELIMINARY DESCRIPTION OF MATERIAL. 



During the first few days of our stay at Murray Island at the North 

 end of the Great Barrier Reef, I sought for the Rhizocephalan para- 

 site of Alpheus amongst the species dwelling in burrows on the reef 

 (such as A. edwardsi), but without any success. On September 30, 

 Dr. H. L. Clark brought to me a few specimens of a dark-coloured 

 Alpheid (Synalpheus brucei, sp. n.), which he had found sheltering 

 amongst the arms of a large Comatulid. One of these was distinguished 

 from its fellows by the possession of numberless little pink egg-like* 

 sacs attached to the thoracic legs and contrasting strongly with the 

 purple colour of the cuticle of the host. These sacs were found to 

 contain Cypris larvae, and so little doubt remained that I was dealing 

 with the creature described by Coutiere under the name of Thyla- 

 coplethus. The next few days proved that, on the south reef of the 

 island of Maer, there lived under every loose stone and within the 

 branches of the living coral a teeming population of crinoids, of which 

 the majority belonged to the species Comanthus annulatum (Bell). 

 Within the living basket formed by the arms of the crinoid, a pair of 

 Alpheids, male and female, were generally to be found, and a small 

 proportion of the whole population were infected with the parasite. 

 So numerous were the crinoids and their commensal Alpheids that they 

 could be depended upon to provide a sufficient quantity of material for 

 the study of the parasite. Only toward the edge of the reef, however, 

 were the crinoids numerous, and as the spring tides waned it became 

 more difficult to locate the crinoid with the water glass, to plunge 

 beneath the water which covered its retreat, to extricate, by means of 

 the brutal hammer or the persuasive hand, the unwilling echinoderm 

 from amongst branching corals, to which it clung with desperate 

 energy, and finally to prevent the escape of the agile Alpheid when the 

 crinoid was raised above water. It was such experiences, however, 

 which gave additional zest to our life at Murray Island. 



I was eventually able to obtain nearly twenty Alpheids infected by 

 Thompsonia in various stages of development. The external sacs 

 varied from tiny bodies barely 0.5 mm. long to those a little more than 

 1 mm. in length and 0.6 mm. in breadth. In the latter stage the Cypris 

 larvae, with their dark eyes, could be plainly made out through the 

 mantle. They were nearly always situated on the limbs. Some 

 Alpheids only showed them on the abdominal swimmerets, where they 

 were to be distinguished from the eggs of the host by their inferior 

 size and different colour, but in the majority of hosts they occurred 

 both on the abdominal and thoracic limbs. On some of the ambula- 

 tory limbs as many as a dozen were found, but in a late stage of develop- 

 ment they were greatly crowded, as is shown in plate 1, figure 2. As 



*The eggs of the Alpheid are, however, green and are larger than the parasite. 



