COPEPODA. 231 



genus Ilia. In Ibla quadrivalvis, side by side with the hermaphrodite form, 

 there is a small complemental male with a large peduncle, but a very reduced 

 capitulum and a diminished number of thoracic limbs, while in Ibla Cumingii 

 a similar male is found side by side with a true female, complete separation of 

 the sexes being here obtained. 



It follows from the above that, in accordance with HOEK (No. 46), in tracing 

 the sexual condition of the Cirripedes, we start from the hermaphrodite form, 

 proceeding to others with dwarf males in company with hermaphrodite 

 individuals, and reach finally a complete separation of the sexes in which 

 marked sexual dimorphism has secondarily developed. The series of com- 

 plemental males, as well as the dwarf males, would thus be derived from the 

 hermaphrodite form by the degeneration of the female genital rudiment. It is 

 perhaps difficult to imagine how the want of complemental males can in the first 

 instance have made itself felt in hermaphroditic forms. But we must bear in 

 mind that, according to F. MULLER, cross-fertilisation appears to be the rule 

 even among the true hermaphroditic Lepadidae. This cross-fertilisation is of 

 the greatest importance in securing the vitality of the race ; but owing to the 

 fixed mode of life of the Cirripedia there is some danger that it may not take 

 place. All such danger is precluded by the development of these dwarfed forms 

 living semi-parasitically on the larger hermaphrodites. The dwarfs, by the 

 atrophy of their ovaries, have become males, and by the attachment to the 

 hermaphrodites ensure cross-fertilisation. We must regard this condition as a 

 partial retrogression in the direction of separation of the sexes, which has again 

 been reached in individual cases in the further course of this development. 



The view sketched above is in opposition to that of CLAUS (No. 8), according 

 to which the sexually distinct forms (e.g., Alcippe and Cryptophialus] have re- 

 tained the primitive condition. From this originally dioecious condition, by a 

 transformation of the females into the large hermaphrodite form, the condition 

 found in most Lepadidae was developed, while the males were only retained in 

 isolated species as complemental males. Consequently the males would be a 

 vestigial remainder from those times when hermaphroditism had not yet become 

 the rule among the Cirripedia. The occurrence of dwarf males side by side 

 with hermaphrodite individuals in a number of forms would be easily explicable 

 by this ingenious theory (not, however, in Scalpellum villomm and S. Peronii). 

 HOEK, however, has pointed out in opposition to it, that, in this case, the dwarf 

 males ought to show a greater resemblance in structure to the Cypris form than 

 they actually do show. The dwarf males in reality appear to be connected by 

 gradual transitions with the complemental males of Scalpellum villosum and S. 

 Peronii, which latter forms are evidently to be derived from the hermaphrodite 

 form. 



The view held by GLAUS would apply to the Rhizocephala if the statements of 

 FR. MULLER and DELAGE were to prove true that complemental males occur in 

 these forms which, throughout life, do not develop further than the Cypris pupa. 

 But this last view, which rests upon the discovery in the cloacal aperture of a 

 young Sacculina extertia of an attached dead Cypris envelope, must still be 

 considered doubtful, and has actually been denied by GIARD. 



6. Copepoda. 



The Copepoda are very numerous and rich in varieties of form, 

 in spite of the simplicity of their body segmentation; they never- 

 theless show morphological characters which, in relation to those 



