24 o DISCOVERY REPORTS 



SUMMARY 



All the Discovery specimens of Gigantocypris examined agreed with Skogsberg's 

 species, G. Mullen, which he described from material collected within the region of 

 the Gulf stream. 



The body of Gigantocypris is enclosed in an almost spherical shell provided with 

 a ventral slit which represents the relatively short length over which the valves are not 

 fused (Fig. i). This bears two antennal notches through which the swimming antennae 

 are extended (Plate XXXIX). 



The body almost fills this enormous shell (Plate XXXIX, fig. i) but it has not enlarged 

 proportionately in all regions. Thus, while the dorso-posterior region between heart 

 and caudal furca has enlarged to a hemisphere concentric with the shell, the limbs 

 have neither enlarged to the same degree nor have they become spaced out. They remain 

 closely packed together in a small antero-ventral region. Again, in the region between 

 the anterior limbs and the heart, the nauplius eye has enlarged to such an extent as to 

 occupy the whole of this region, while the paired eyes have shrunk to two small sacs 

 and the frontal organ has dwindled to a minute tubercle on the lower face of the 

 nauplius eye (Fig. i and Plate XLI, figs, i, 3). 



From its shape and size it can be deduced that Gigantocypris cannot move quickly 

 through the water. Despite this, it feeds on active food such as giant sagitta, fast 

 swimming copepods and young fish. It is suggested that it is a sedentary feeder grasping 

 live prey by its mandibular palps which are extended through the antennal notches 

 (Plate XXXIX, fig. 6). 



The movement of the mouth and mouthparts towards the antennal notches is brought 

 about by a hydrostatic system controlled by the criss-cross musculature of the flexible 

 dorsal body wall of the trunk region. Contraction of this system of muscles forces the 

 body fluid from the posterior into the anterior region of the body and so extends the 

 mouth downwards. The reverse process is brought about by the dorso-ventral body 

 retractors which are attached ventrally to the adductor tendon and dorso-laterally to 



the shell (Figs. 8, 9). 



There is a nervous control which corresponds closely to this muscular system. It 

 consists of a single neurone in the dorsal side of the heart which sends out two axons 

 laterally that emerge from the heart on either side as a cardiac nerve. These extend 

 downwards and then bifurcate into two branches. The posterior sends connexions to 

 the circular and longitudinal muscles of the dorsal trunk region and then continues 

 downwards to the posterior chain system. The anterior sends connexions to the dorso- 

 ventral body retractor muscle and then continues downwards to the hinder part of the 

 nerve ring (Fig. 9). 



Further evidence is given from the study of an embryo that the circular muscles of 

 the dorsal dome of the trunk region are of ectodermal origin (Fig. 2). 



The body "floats" inside the spherical shell, being held in position by the adductor 



