66 



J.A. ALLEN, H.L. SANDERS AND F. HANNAH 



4-, 



3- 



E 

 E 



o> 



CO 



2- 



1- 



T 1 1 1 1 1 1 1 1 1 1 1 



1 5 9 



No. of teeth 



Fig. 103 Yoldiella jeffreysi. The relationship of the number of 

 hinge-teeth to shell length of a subsample from Sta. 316, West 

 European Basin. The left and right limits of each bar indicate the 

 number of anterior and posterior hinge-teeth respectively. 



anterior of the body, thereafter describing an 'S'-shaped bend 

 before continuing as an anticlockwise loop which partially 

 overlies the first. Thereafter it runs parallel to and immedi- 



Fig. 102 Yoldiella jeffreysi. a, lateral view of a 

 right and left valve of a specimen from the 

 Jeffreys collection labelled Leda lata (USNM 

 No. 199695, Valorous Expedition, Sta. 9). b, a 

 dorsal and lateral view from the right side of a 

 specimen from Sta. INCAL DS 06, West 

 European Basin. (Scale = 1.0 mm). 



ately below the dorsal margin to the anus (see Fig. 107). A 

 typhlosole is present along its entire length. The configura- 

 tion of the hind gut on the right side of the body, despite 

 being somewhat variable is characteristic of this species (Fig. 

 108). It can be very simply visualized as a doubled length of 

 rope making one and a half turns (Fig. 107). In this species, 

 more than any other we have examined, there is a consider- 

 able variation in the course of the hind gut which can easily be 

 resolved by reference to the rope analogy (Fig. 107). It would 

 appear that this manner of accommodation of a long hind gut 

 is particularly susceptible to distortion during development, 

 possibly because of displacement due to the large size of the 

 stomach and the anterior arrangement of the digestive diver- 

 ticula or possibly because the loop tends to impede its own 

 development in this particular configuration (Fig. 105). 



The nervous system is well-developed with moderately 

 large 'club'-shaped cerebral ganglia, slightly smaller elongate 

 visceral ganglia and large, oval, pedal ganglia. Dorsal to the 

 latter are large, round, statocysts, filled with refractile gran- 

 ules. The visceral ganglia lie some distance anterior to the 

 posterior adductor. 



The populations from the different basins have similar 

 internal morphologies. 



Sexes are separate and the gonads overlie other internal 

 organs. Gonad development was followed in two Incal 

 samples (DS07, DS09; West European) and one Discovery 

 sample (8521 No. 1, Cape Verde). In West European speci- 

 mens the number of ova ranged from 160 (108 um maximum 

 diameter) in a female 2.1 mm long to 350 (144 \xm max. 

 diameter) in a female 3.1 mm long. The females from the 

 Cape Verde Basin (2.2, 2.5 & 2.7 mm total length) contained 

 fewer eggs 41-103 but these had a slightly larger size range 

 130-156 (Jim. 



In July/August samples, maturing gonads had become 



