SYSTEMATIC ACCOUNT 155 



in Idotea emarginata (Fabr.) the rudiments of the lamellae appear as four pairs of buds on the inner 

 ventral margins of the coxae of the first four free thoracic segments. In the breeding female these 

 buds develop into large, partly chitinized lamellae, lying ventrally to the coxae and overlapping one 

 another to form a complete marsupium (Sheppard, 1939); there are no ventral coxal plates to the 

 coxae of those limbs bearing lamellae, these appear to replace them. 



With one exception, the published descriptions of the species of Edotia make no mention of any 

 variation from this type of marsupium, but the breeding females of this genus in the Discovery 

 collections all show some modification of the normal form. This modification is most marked in 

 E. ocidata Ohlin, where a condition is reached, the nearest parallel to which is seen in the Sphaero- 

 midae, where brood pouches are formed by involution of either the ventral or the lateral integument. 

 The single exception just mentioned is found in Nordenstam's (1933) description of E. tuberculata 

 Guerin-Meneville { = E. oculopetiolata of this paper). 



Nordenstam (1933, p. 95), in describing the four pairs of brood lamellae of this species, said: 'they 

 are all fused with each other by a thin chitinous tegument', and 'the anterior margin of the small first 

 pair (belonging to the first pereiopods) is not fused anteriorly with the sternum '. Finally, of the 

 fourth pair he said: 'its posterior margin is fastened to the sternum by a thin chitinous tegument.' 



From this statement it would appear that the brood pouch, in this species, was a closed pouch 

 communicating with the exterior by a small anterior opening only; this, however, is not the case. 

 I have examined a breeding female of E. oculopetiolata and find that contrary to Nordenstam's 

 observations the four pairs of brood lamellae are quite distinct, though modified. They differ from 

 the common isopod type of large overlapping, partially chitinized, lamellae. Each lamella, when 

 viewed from below, can be seen to be highly chitinized and appears to meet its fellow in the mid- 

 ventral line. The posterior chitinized edge of one meets the anterior chitinized edge of the one im- 

 mediately behind, each of the first three pairs of lamellae possessing, in addition, non-chitinous 

 extensions of the posterior margins, which lie underneath (dorsally to) the chitinous anterior part of 

 the succeeding pair (Text-fig. 56). 



The fourth pair of lamellae are chitinized throughout and are longer than broad (Text-fig. 5 c). 

 The inner margins of the lamellae also possess non-chitinized extensions, which are short and bent 

 dorsally upwards at right angles. The outer posterior angle (Text-fig. 5^, /) in each of the three 

 anterior pairs of lamellae is produced into a strong lobe, which lies over the outer corner of the 

 anterior margin of the lamella immediately behind, and forms an interlocking device. The strongly 

 chitinized lamellae, together with the added rigidity produced by this interlocking, appear to be 

 sufficient to prevent ventral sagging of the pouch and the consequent escape of its contained young, 

 which develop between the lamellae and the ventral body wall. This wall is soft and becomes pushed 

 up dorsally to lie very close to the dorsal body wall, carrying the ventral nerve cord with it into close 

 proximity with the alimentary canal. A similar type of marsupium to that of E. oculopetiolata is also 

 found in the breeding female of E. corrugata sp.n. 



In the females of E. oculata Ohlin the modification of the brood pouch is carried farther than in 

 other species of the genus. Among the Discovery specimens of this species there are a number of 

 breeding and non-breeding females. In the breeding females the first four pairs of coxal plates 

 (Text-fig. 6c), when viewed from below, very closely resemble those of the male (Text-fig. 6e); that is, 

 they are large and nearly meet in the mid-ventral line. As in the male, the outer parts of these plates 

 support the ventral integument, but the inner distal ends take on the function of acting as brood 

 lamellae. 



In the non-breeding females on the other hand, the first four pairs of coxal plates are short and 

 comparatively narrow, with a broad unchitinized area of ventral integument between their distal ends 



