280 PHYLUM ANNELIDA 



mounts guard over them for more than a hundred days. The re- 

 markable Branchellion on the Torpedo has eleven pairs of leaf-like 

 respiratory plates on the sides of its body, and so has the related 

 Ozohranchus jantseaniis, a parasite of a river turtle in the Jantsekiang. 

 One of the strangest habitats is that of Lophobdella, on the lips and 

 jaws of the crocodile. 



Classification. — 



Family i. Rhynchobdellida, in which the fore part of the 

 pharynx can be protruded as a proboscis. There is an anterior 

 as well as a posterior sucker. The blood plasma is colourless. 

 The ova are large and rich in yolk ; the embryos are hatched 

 at an advanced stage, and soon leave the cocoon, which contains 

 no albuminous fluid. 



e.g. Clepsine, Pontobdella, Branchellion. 



Family 2. Gnathobdellidaj, in which there is no proboscis, but 

 the pharynx usually bears three tooth-plates. The mouth is 

 suctorial. The blood plasma is red. The ova are small 

 and without much yolk ; the embryos are hatched at an 

 early stage, and swim about in the nutritive albuminous fluid 

 of the cocoon. 



e.g. Hirudo, Hctmopis, Hcemadipsa, Aulostoma, Nephelis. 



Family 3. Acanthobdellidas. By itself is the .Siberian fish parasite 

 Acanthobdella, which has rows of setae on the first five segments, 

 a spacious coelom, and other peculiarities. 



ASSOCIATED CLASSES 



The seven classes which follow are associated more or 

 less with Annelid worms, though the relationships are not 

 yet clear. 



Class Ch^tognatha. Arrow-worms 



The. pelagic Sagitta and Spadella require a class by themselves. 

 They may be regarded as Annelids with three segments. The trans- 

 lucent body, which may be nearly 3 in. long, but is usually much less, 

 has three distinct regions — a head bearing a ventral mouth with spines 

 and bristles (whence the name Cha^tognatha), a median region with 

 lateral fins, and a trowel-like tail. The nervous system consists of a 

 supra-CESophageal ganglion in the head, a sub- oesophageal about the 

 middle of the body, long commissures between them, and numerous 

 nerves from both ; it retains its primitive connection with the epi- 

 dermis. There are tw'o eyes and various patches of sensitive cells. 

 The food canal is complete and simple, and lies in a spacious ciliated 

 body cavity. Corresponding to the external divisions, the cavities 

 of the head, body, and tail are distinct, being separated from one 

 another by septa ; a longitudinal mesentery supports the gut and 

 divides the cavities into lateral halves. 



