ANNELIDS— BRISTLE-FOOTED WORMS. 671 



But too much stress must not be laid upon this circumstance, seeing that other 

 marine groups of invertebrated animals, like the Mollusca and Hemichordata, 

 may also be developed from ciliated larvae not very different in structure from 

 those that are met with in the Vermes. 



The Vermes may be divided into the following six classes : — 1. Annelida 

 (earthworms, leeches, etc.); 2. Gephyrea ; 3. Nematohelminthes (thread- 

 worms); 4. Rotifera (wheel-animalcules) ; 5. Nemertinea; 6. Platyhelminthes 

 (flat-worms). 



THE RINGED WORMS. 



Class Annelida. 



To this class belong those highly specialised worms whose organisation has 

 been briefly referred to above. The body consists of a series of rings or 

 segments, each of which is substantially similar to the one behind and the 

 one in front of it. The alimentary canal traverses the body from end to end, 

 and beneath it lies the nerve-chord, consisting of a double thread united in 

 each segment to form a ganglion or swelling, and connected in front with the 

 brain, one of the threads passing on each side of the oesophagus. Each seg- 

 ment, too, typically contains a pair of excretory tubes or nephridia, and the 

 circulatory system consists of longitudinal vessels, connected by transverse 

 branches. The most highly organised members of this class, namely, the 

 Choetopoda, are structurally not very far removed from the Arthropoda 

 (crustaceano, insects, centipedes, etc.). The chief distinctions between the 

 two groups are, however, three in number. In all the Arthropods, except 

 Peripatus, the legs are jointed, and at least one pair of them has become 

 modified to act as jaws. But in the Chfetopoda, although there may be 

 distinct leg-like processes, these are never jointed, and the jaws, when 

 present, are merely horny teeth developed in the mouth. Again, in the 

 Arthropods the walls of the heart, or dorsal blood-vessel, are perforated by 

 slits, by means of which the blood makes its way back to that organ, 

 whereas no such slits are present in the corresponding blood-vessels in the 

 Annelids. The Annelida are divisible into the CHiETOPODA and Hikudinea . 



The Bristle-Footed Worms — Sub-Class Chfetopoda.— At the head 

 of the Vermes stand the species known as the Bristle-Footed Worms 

 or Chfetopoda, which in many details of their organisation approach the 

 Arthropoda. For example, in the typical members of the class the body 

 consists of a series of approximately similar .segments, each of which is 

 furnished at the sides with two rows of bristles, and very often with limb- 

 like prominences, known as a parapodia. There is a distinct head, bearing 

 above the mouth, a prostomium or kind of enlarged upper lip, and often 

 furnished in addition with one or more pairs of tentacles. The alimentary 

 canal traverses the body from end to end ; and beneath it lies the double 

 nerve chord, which is in front connected with the brain, placed in the head 

 above the mouth. 



Leaving aside for the moment some of the less important types, the 

 Chsetopoda may be said to fall into two orders, known as the PoLYCHiETA 

 and the OLiGOCHiETA. 



The Polychseta have a conspicuous head, usually bearing feelers and 

 gills, and the segments of the body are furnished at the sides with 

 limb -like processes, usually supplied with a rich armature of bristles. 



