cxlvi Introduction. 



are viviparous. In probably every case, except perhaps the last- 

 mentioned, the impregnated ovum takes the shape of a bilaterally 

 symmetrical and ciliated larva, provided with more or fewer pro- 

 visional organs. Where the ova are few and large, and during 

 development, as in Pteraster^ EcJmiaster, and some other Aste- 

 roidea, protected by a dorsally or ventrally formed maternal marsu- 

 pium, the provisional organs may be reduced to a minimum, and the 

 development becomes almiost direct. But in most cases the Echi- 

 nodermata go through a very well-marked metamorphosis, which 

 often has more than one larval stage. The distinctive character 

 of the metamorphosis appears to be the possession by the larvae 

 of at least a mouth and pharynx, which, whether absorbed or cast 

 off, is never converted into the corresponding organs of the perfect 

 Echinoderm developed inside of the provisional organism. The 

 mass of more or less differentiated sarcode, of which the larva or 

 pseudembryo as opposed to the Echinoderm within it, is made up, 

 always carries upon its exterior certain bilaterally-arranged ciliated 

 bands, by the action of which the whole organism is moved from 

 place to place j and it may be strengthened by the superaddition to 

 it of a framework of calcareous rods. In the larval Asteriae known 

 as Bipinnariae, the provisional organism, when discarded, as it is, by 

 the young Echinoderm, has been observed to retain an independ- 

 ent vitality for some days ; but it has not been observed to give 

 rise to any second zooid ; and when we consider the greater tenacity 

 of life which isolated portions of these animals [Asteriae and Holo- 

 thurioidea) have often been observed to possess, the history of a 

 Bipinnaria might appear to be analogous rather to the ecdysis of a 

 Crustacean, or the metamorj)hosis of a Dipterous Insect, than to 

 true metagenesis. 



The history, however, given by Professor Grube (Monatsbericht. Konigl 

 Akad. Wiss. zu Berlin, 12 Marz, 1868) of the discovery of an Echinoid, 

 Anochanus sinensis, which, while 2:>ossessing the external organs, such as 

 spines, pedicellai-iae, and ambulacra, characteristic of adult Echinodermata, 

 had neither genital glands nor orifices ; but in place of them an apically 

 situated sac containing several young Echinoidea in very various stages of 

 development ; will, if confirmed, cause us to demur to the view adopted 

 below (pp. 147, 153), to the effect that in Echinodermata metagenesis 

 with alternation of generations, as distinct from metamorphosis, is not to 

 be found. At present, as but a solitary specimen has been observed, it 



