1903. J FROM EAST AFRICA AND ZANZIBAR. 359 



Another point of importance is the presence or absence of a 

 labial armature, that is to say, of a ring or plates on the labial 

 cuticle composed of a compact mass of minute hooks or rods. 

 There is some reason for dividing the group into those which 

 possess and those which do not possess this armature, but still I 

 think that an absolute dichotomy of this kind presents difficulties. 

 Firstly, a rudimentary labial armature is of some inconvenience 

 in classification. Bergh has described such an instance in Platy- 

 doi-is variegaia ; and the animal described below as Thordisa 

 crosslandi has two small areas on the labial cuticle which cannot 

 be called plates, but consist of a loose collection of minute rods. 

 In Plaiydoris piolchra the labial cuticle is strengthened with 

 similar rods, distributed through its extent, but not collected into 

 rings or plates. Secondly, though Discodoris B. is a well-defined 

 genus, the same can hardly be said of the family Discodorididre, the 

 genera of which mostly agree only in having a labial armature. 

 Thus Fracassa and Carminodoris appear to be akin to Trippa, 

 Hoplodoris to Platydoris, and Halla to Chromodoi'is. It would seem 

 that the more primitive forms of the Cryptobranchiata are those 

 which have a labial armature and some differentiation of the 

 teeth, such as occurs in Chromodoris and Cadlina, and that the 

 forms with no labial armature and uniform hamate teeth are 

 more recent developments. If this be so, it is easy to under- 

 stand that in many groups a few forms have survived in Avhich 

 the labial armature has persisted. Thus Fracassa is practically a 

 Trippa which has preserved this character, and it appears to me 

 that its analogies to Trippa are greater than those to Discodoris. 



(e) Considerable use has been made by Prof. Bergh of the 

 reproductive organs as a character for classification*. That great 

 weight must be attached to important variations in these organs 

 — such as the i^resence of one or two sperm athecte — is obvious ; 

 but while fully admitting the necessity of examining the internal 

 anatomy and the futility of describing only the external characters 

 of Nudibranchs, it must also be admitted that it is not con- 

 venient to found genei-a of fairly large animals upon minute 

 internal characteristics which can only be discovered by an expert 

 microscopist, and by him only in a well-preserved and mature 

 specimen. Such a criterion seems desirable only if it is of great 

 anatomical importance. But what are the variations presented 

 by these organs in the Cryptobranchiata? (i.) Accessory organs 

 are sometimes present. These are generally accompanied by 

 other characters of systematic importance as in Kentrodoris and 

 Asteronotus. (ii.) The male branch of the hermaphrodite system 

 sometimes broadens and sometimes does not into a dilatation 

 called the prostate. It does not appear that this difference is 

 generally accompanied by other characters of importance : e.g. &, 



* But see System, d. Nud. Gast. p. 1088. "Bei dem jetzigen Standc unserer 

 Kenntniss . . . . ist es sehr gewagt eine systematische Gruppirung derselben zu 

 versucLen. Es ist es um so mehr als der generische Werth der bei der hicr ver- 

 guchten Gruppirung dem genital System, besonders den verschiedenen ConforTna- 

 tionen seiner Ausfuhrungsgange, beigelegt ist, als solcher kaum sieher gestellt ist." 



[7J 



