18 



NATURAL HISTORY OF ARTHROPODS. 



of the female, those of Cryptophialus minutus closely resembling those of Alcippe, which 

 was formerly placed among the Thoracica, and showing far more resemblance to those 

 of Ibla and Scalpellum than do the females. 



But little is known of the embryology of this group. Alcippe hatches as a true 

 nauplius and subsequently passes into the pupa state, closely resembling that of the 

 Thoracica, while in the other forms a free nauplius stage is absent. The embryo is at 

 first oval, but soon two anterior processes, the rudimentary antennas, appear. The larva 

 next passes into the free cypris stage, creeping about freely within the mantle of 

 the mother. The subsequent stages are not known. 



The known species of Abdominalia have the following distribution : Cryptophalius 

 comes from the Chonos Islands, Southern Chili ; Alcippe from the English coast, and 

 Kochlorine from the Mediterranean. 



OHDER III. - - CIRRIPEDIA RHIZOCEPHALA. 



These degraded Cirripeds, in their adult condition, live as external parasites upon 

 the abdomen of the higher Crustacea, and by degeneration have so completely lost 

 every trace of an articulate structure that were we ignorant of their development, their 

 affinities with the Crustacea would not be suspected. In shape they are sack-like, more 

 or less modified in form by pressure between the thorax and abdomen of the host. 

 From one side arises a tubular process which penetrates the host and frequently divides 

 up into numerous root-like branches. This tube is in reality the mouth, and through it 

 the juices of the crab pass into the body of the parasite. Nearly opposite the mouth 

 is an opening through which the eggs pass out. The whole body is enclosed in a 

 thick muscular mantle, which is never calcified. The internal organs are principally 

 reproductive, the animals being hermaphroditic, while the digestive organs are fre- 

 quently entirely absent. No traces of a nervous system have as yet been found. 



When we study the embryology of the group we recognize 

 at once its affinities. From the egg there hatches a nauplius 

 resembling that of the true barnacle in having frontal horns, 



O O ' 



while the hinder end of the body ends in two points and the 

 mouth and alimentary canal are Avanting. After several moults 

 the cypris stage is reached and the larva after swimming freely 

 for a while attaches itself to the abdomen of some crab, and 

 there undergoes the retrograde metamorphosis which results in 

 the degraded adult. The Rhizocephala are usually regarded 

 as the lowest of the Crustacea, but Kossmann, whose account we 

 are following, places them as they are here classified. 



Six genera and about forty species of Rhizocephala have 

 been described, but the distinctions between the various forms frequently seem unim- 

 portant, and the characteristic which is most emphasized in the diagnoses 

 is the host, it being assumed that the parasites of different species of 

 crabs must of necessity be distinct. Several forms occur in North 

 American waters, but they have not been studied. We figure two 

 species : Peltogaster pagiiri (Fig. 23), which, as its name indicates, 

 is parasitic upon the hermit crabs, and Lernceodiscus porcellance (Fig. 

 24), as attached to the abdomen of a porcelain crab. The roots, which FIG. 24. 

 are well shown in the former figure, are said to have the power of growth 



FIG. 23. Peltogaster pag- 

 uri, showing root-like 

 mouth - parts. En- 

 larged. 



