NATURAL HISTORY. 



XOKTHIA TITHICOLA. 



well-developed foot organs, which lead a roaming life, the exceptions being very few, and which 

 are carnivorous and predaceous they are the Errantia : secondly, those which live in protecting 

 tubular structures, and which have feebly developed feet, and are called the Tubicola. 



In examining these many -bristled Worms it is advisable to employ certain descriptive terms. 

 Thus, the first segment of the body is called the prostomium, and the mouth opens on it : the second 

 is the peristomium. When the three front segments are united, or when they differ from those 

 which come after, they are called the head or cephalic segments ; but when this is not the case 



the Worm is said to be acephalous. The head has 

 various appendages according to the genera. An- 

 tennae are soft filaments varying in number from 

 one to five, and they arise directly from the head, 

 are not retractile, and are usually jointed at the 

 base. Sometimes palpi exist, and they are soft, 

 entire, or jointed processes, arising from the sides 

 of the mouth. The tentacles are soft, bristly, or 

 thread-shaped, non-retractile processes, which arise 

 from each side of the segments of the head in pairs, 

 and spread laterally. They are often very long, 

 and are contractile in the acephalous genera. 



The mouth is underneath the head, and is a 



A, cephalic segment; B.iuouth, &c.; o, tentacular cirri: 6, upper lip. 1'OUlld Or transverse Opening to the gullet. 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 horizontally. It is almost always furnished with a proboscis in the 

 cephalous tribes ; that is to say, the oesophagus or gullet can be protruded. It consists of two seg- 

 ments, 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 in some of the others, the thoracic segments are distinguished by peculiarities in 

 their structures 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. This has no setigerous 

 feet, and no soft appendages ; but more commonly a pair of soft 

 filaments, called styles, project behind. The vent is terminal and 

 central. The segments have appendages on either side, and the 

 principal is a lobe, which is called the foot, or parapodium. 



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

 on either side of a segment. It supports the bristles, which are, as 

 it were, sheathed by it, and it is a basis of attachment 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 ventral. 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. Taking the 

 biramous foot of one side of a segment of Nephthys longisetosa as an example, the upper 

 and ventral lobes are seen to be wide apart, but to be connected. The bristles of the two 

 lobes are long hair-like setse ; the cirri are two curved booklets projecting downwards from 

 each lobe, and besides these there is a kind of flap behind the bristles, which probably 

 is a rudimentary branchia. The bristles are of four kinds in these Polychaeta, the spine, 

 which is subulate, straight and tapering from the base to the apex. It is placed in the 

 midst of a bundle of bristles. The spinet is a hook or fork, and is only found in a few 



FOOT OF XEPHTJIYS. 



