V CRUSTACEA— ONTOGENY 389 



form on the mantle-fold. The Cypris shell is thrown off, and the paired eyes 

 disappear. 



The history of the development of the attached and parasitic Cirripedia is in 

 many respects exceedingly interesting. The Cypris-lilce larva shows, apart from the 

 absence of posterior antennre and mandibular feet, a distinctly Eutomostracan 

 character. Its 6 typical hiramose feet recall the swimmerets of the Cojicpoda. The 

 remarkable transformation of the free-smmming Cypris-like larva into the attached 

 sexual animal must be referred to its adaptation to an attached mode of life ; this 

 adaptation consists in the formation of a hard shell or framework serving for protec- 

 tion, the transformation of the swimmerets into tendril-like feet suited for bringing 

 food within reach, and the reduction of the paired eyes which are of no great use to 

 the adult animal. A commencement of this adaptation to the attached mode of life 

 may perhaps be seen even in the Cypris stage, viz., in the degeneration of the 

 posterior anteunie and mandibular feet, which in the Nauplius larva had played an 

 important part, especially as organs of locomotion, whereas in the Cypris-like larva 

 the trunk feet serve that purpose. The transformation of the Cypris-like larva into 

 the parasitic Sacculina involves far more radical changes than those which take 

 place in the development of the Lepadidce and Balanidcc. The development of 

 Sacculina may be described as a strikingly retrogi'essive metamorphosis. To explain 

 the reappearance of the typical Nauplius and Cypris-like larvte in the course of 

 development, notwithstanding the degradation of the adult animal, it is necessary 

 not only to emphasise the power of inheritance but to remember that free-moving 

 young forms are extremely useful to parasites for the infection of new hosts and 

 the maintenance of the race. Nevertheless, even in the free-swimming larvfe of 

 Sacculina, we recognise distinct signs of degeneration such as the absence of an 

 alimentary canal. This degeneration could take place without damage to the 

 maintenance of the race, because the adult Sacculiiuo, in consequence of its exceed- 

 ingly favourable conditions of nutrition, can give its eggs for their development so 

 much nutritive yolk that the larvfe proceeding from them are under no necessity of 

 obtaining food from without. The occurrence, at first sight so remarkable, of an 

 endoparasitic stage in the development of the Saccttlina is not difficult to under- 

 stand, for by passing tlirough such a stage the parasite avoids the danger of being 

 thrown off by the moulting of the host. 



Larval History of the Euphausidae (Order Schizopoda, Fig. 262). 



1. Nauplius Stage. — Typical. 



2. Metauauplius Stage. — The masticatory ridge of the mandible develops, while 

 the mandibular foot itself is reduced. The upper and under lips form. Behind 

 the mandibles the rudiments of the 2 pairs of maxillfe and of the 1st pair of thoracic 

 feet (maxillipedes) appear as buds. The cephalothoracic shield is distinctly 

 developed, and the paired eyes first appear. 



3. Calyptopis Stage (somewhat corresponding to the Frotoxocea stage of Penccus) 

 (B, C). — The thorax and abdomen are demarcated, and the latter already elongated. 

 Segmentation appears in the thorax, and later in the abdomen. No new extremities 

 except the last pair of pleopoda (uropoda) begin to form. 



4. Furcilia Stage. — The paired eyes become stalked. The most anterior pairs of 

 thoracic and abdominal feet begin to form in succession from before backward. 



5. Cyrtopia Stage. — The antennfe are transfonned and no longer serve for 

 locomotion. The posterior pairs of thoracic and abdominal feet and the gills 

 appear. 



6. Post-larval Stages. — The adult animal is gradually formed, and the caudal 

 fin definitely developed. It is hardly necessary to remark that all the thoracic feet 



