VEEMES. 57 



Class III. CHJETOGNATHA. 



CESTHELMINTHA. 



Free, elongate, transparent animals, with preoral setae and a 

 fin-like tail, but without limbs. Hermaphrodite. No metamor- 

 phosis. 



The body is fringed on each side by a striated fin -like mem- 

 brane. The intestine is a straight tube. The nervous system 

 consists chiefly of a single abdominal ganglion, sending backwards 

 and forwards two pairs of lateral branches. The eyes are two 

 pigment-spots. The embryo is non-ciliated, and its develop- 

 ment is, in some respects, peculiar. 



This is a very aberrant form, and has even been regarded as a 

 vertebrate. Forbes placed it among the Mollusca. There are six 

 species, varying from half to one inch in length, mostly European, 

 of the one genus 



Sagitta. 

 Class IV. GEPHYREA. 



SlPUNCULACEA. EHYNGODEA. 



Body cylindrical, with a thick coriaceous skin, often indistinctly 

 ringed. No limbs. Head not distinct from the body, often pro- 

 duced into a proboscis. Sexes distinct. 



There are no respiratory organs, and sometimes no vascular 

 system. The mouth, with or without tentacula, is placed at the 

 end of a retractile proboscis. The anus is either at the base of 

 the proboscis, or is at the posterior extremity of the body. Eye- 

 specks are present in certain Sipunculidas. A Planaria-like form 

 of male occurs in Bonellia. These animals have ciliated free- 

 swimming embryos, not unlike Rotifera, to which Huxley con- 

 siders them closely allied. In the subsequent growth only part 

 of the larva is concerned in the development of the adult (an 

 alternation of generation, or metagenesis). 



The Gephyrea, of which there are about 120 known species, 

 vary from half to eight or more inches in length. They live 

 among rocks or seaweed, or are sometimes found buried in the 



Formerly classed with the Echinodermata, they are now di- 

 vided into two, somewhat artificial, orders Inerines (without 



