WORMS, ROTIFERS, LEECHES, POLYZOA 83 



It has usually a plain margin. In the acephalous genera it is 

 terminal, and has external tentacles, but there are no jaws, and 

 in the cephalous it is nearly terminal and looks forward horizon- 

 tally. It is almost always furnished with a proboscis in the cepha- 

 lous tribes ; that is to say, the oesophagus or gullet can be 

 protruded. It consists of two segments, and can be put forth at 

 pleasure by a process of turning inside out. It is often armed 

 with horny jaws, in opposite pairs, or is roughened on the surface 

 with horny prickles ; or it may be covered with pimples or be 

 plain. The head is succeeded by the " thoracic segments," and in 

 the cephalous genera there is but one of them. It is naked and 

 has no appendages. But in the acephalous genera and some of 

 the others the thoracic segments are distinguished by peculiari- 

 ties in their structure and appendages. They may be fleshy and 

 contain most important organs, and the branchiae are often limited 

 to them. The abdominal segments complete the body, vary in 

 number, are alike, and lessen in size, the last being the anal. 



" The so-called foot, or parapodium, is a pimple-shaped pro- 

 jection on either side of a segment. It supports the bristles, 

 which are, as it were, sheathed by it, and it is a basis of attach- 

 ment for the branchiae and soft, setaceous filaments, called cirri, 

 resembling tentacles. The foot may be in one lobe, or there may 

 be two lobes ; one upper or dorsal, and the other lower or ven- 

 tral. These lobes, also called branches, are more or less apart, 

 and when there is but one branch or lobe the foot is said to be 

 uniramouS; and when there are two biramous. The branchiae in 

 the cephalous worms are attached to the base of the foot on the 

 upper or dorsal side, and are either restricted to a certain num- 

 ber of segments or they are found on all. They are either 

 arborescent, combed on one side, flat, and veined, or they may 

 be filamentary." x 



It will be seen from the above outline of some of the anatomical 

 details that the Polychseta are highly specialised animals. They 

 are very numerous, have a wide geographical distribution, and 

 are most interesting creatures. The majority of the Wandering 

 Worms, or Enctntia, are active, predatory animals, have a distinct 

 head, carrying eyes, tentacles, and usually tentacular cirri. 



Most visitors to the seaside are familiar with the little mounds 



1 Dr. P. Martin Duncan, F.R.S. 



