MORPHOLOGY OF THE VALVIFERA 149 



The following evolutionary series illustrates the way in which the change in form and position of 

 the penis may have occurred. 



(i) In Idotea indica (Text-fig. 2) the penes are united at their base and spring from the articular 

 membrane between the last thoracic and the first abdominal segments. 



(ii) In Edotia oculata (Text-fig. i b) the fusion of the penes is more complete, and their base is 

 nearer the abdominal segment, the anterior margin of which is slightly excavated to receive it. 



(iii) In E. ocidopetiolata (Text-fig. i c) the excavation is much deeper and narrower and the fused 

 base of the penes (indicated in the figure) is reduced in size. 



(iv) In Pseudidothea bonnieri (Text-fig. i d) the penis issues from a median aperture in the sternum 

 of the first abdominal segment, just anterior to the appendages. The double nature of the penis is 

 still indicated by the presence of two ducts and the distal cleft. I have been able to examine a male 

 specimen of Amesopous richardsonae Stebbing and find that in this species from another family the 

 penis is single. 



(v) In members of the Astacillidae the penis is single though the vasa deferentia remain paired and 

 open separately at its distal extremity. 



This series of modifications appears to be due to the narrowing of the body, and, in particular, the 

 narrowing of the abdominal region. A small portion of the articular membrane between the last 

 thoracic and first abdominal segment has gradually become pinched in, and finally cut off by lateral 

 compression of the sternum of the first abdominal segment. This has resulted in a consequent 

 narrowing and fusion of the penial filaments. 



SYSTEMATIC ACCOUNT 



Family IDOTEIDAE Dana 



Subfamily Idoteinae, Dana 1852; Miers, 1881 



Genus Idotea Fabricius, 1798 



Idotea indica Milne Edwards, 1840 (Text-figs. 2, 3 a-d) 



I. indica Milne Edwards, 1840, p. 131. 



/. latreillii Guerin Meneville, 1843, p. 32. 



/. indica Miers, 1881, p. 50, pi. 2, figs. 4 and 5; Stebbing, 1902, pp. 62-3; Stebbing, 1910, p. 432. 



Occurrence. At anchor, Houtjes Point, Saldanha Bay, 25. v. 30, 12 m., i^ and 1$ (non-breeding); ectoparasitic 

 on Trigla capensis; Walvis Bay, 9. ix. 26, icj. 



Remarks. Judging by the measurements given for this species in Stebbing's paper (1902, p. 63), 

 the specimens in this collection are not full grown; the larger of the two males measures 19 mm. in 

 length and 7 mm. in greatest breadth, the female is considerably larger, being 24 mm. in length and 

 7-5 mm. in breadth. The female is in the non-breeding condition and bears five pairs of small brood- 

 plates on the coxal plates of the second to the sixth thoracic limbs. 



There is little to add to the already pubHshed accounts of this species; Miers (1881, p. 50) stated 

 that the last pair of thoracic appendages have 'their penultimate joints thickened and considerably 

 elongated'; Stebbing (1902, p. 63), on the other hand, considered that 'the last peraeopods are not 

 very strikingly larger than the penultimate pair' and suggested that this may be a character of the 

 male sex. My own observations agree with those of Miers, namely, that in both sexes the last pair of 

 thoracic limbs (Text-fig. 3 a, e) are considerably longer than the penultimate pair (Text-fig. 3 c, d). 

 The longitudinal medio-dorsal furrow on the metasome mentioned by Guerin-MeneviUe (1843, p. 32) 

 is present but not very deep. 



