Glass 3. Acephala. 



315 



3 Tte Freshwater Mussels {Anodonta) are large, egg-shaped, 

 tMn-shelled LameUihranclis, abundant in freshwater. The numerous eggs 

 hatch in the outer giUs of the female, and the larvas 

 escape by the cloacal aperture. These lai-vse are provided 

 with long, sticky thi-eads, which float in the water and 

 adhere easily to passing Fish; when this occurs the young 

 Mussel attaches itself firmly to the Fish by means of the 

 teeth present on the lower edge of each valve of the shell ; 

 it is then covered by a growth of the skin, and for a time 

 leads a parasitic life upon the Fish, which it forsakes 

 again later. The River Mussel (Unio) and the River 

 Pearl Mussel {Margaritana margarUifera), which are com- 

 mon in England, are allied forms. The latter manufactures 

 some of the pearls of commerce. The true Pearl Mussel 

 (Meleagrina ma/rga/ritifera), which forms the best pearls, belongs 

 to another family of LameUibranchs ; it is found in the Indian 

 and Pacific oceans. 



4. The Gaper (My a arenaria) is distinguished by the 

 possession of a very long syphon (foi-med, of course, by the 

 fusion of two tubes), and by the way the shells gape open 

 behind; the edges of the mantle are concrescent to a large 

 extent. It occui-s on the beach on the coast of Britain, buried 

 more than a foot deep. 



5. The Ship-worm (Teredo navalis) is a vermiform, 

 elongate LameUibranch, in which the edges of the mantle 

 have undergone concrescence to a large extent ; and a pair 

 of very small shell- valves, which are not connected by a 

 ligament, are present anteriorly ; and two partially separated 

 syphons posteriorly. It is a mai-ine form occuiTuig in wood 

 (piles or ships'), in which it bores long tubes, lined with a 

 calcareous secretion; the external apertui-e and the portion of the 

 tube next this are naiTOw, and were formed by the animal when 

 young ; further in it is wider and cylindi-ical : the animal is 

 tmable to leave the tube, its anterior end is in the innei-most 

 portion of the tube, the syphons at the mouth : common on the 

 coasts of Europe : very destructive. In the allied genus, Pholas 

 (P id dock), which boi-es into hmestone, wood, etc., the body is 

 shorter, the shell better developed than in Teredo ; it is phos- 

 phorescent : present in European seas. 



Class 4. Cephalopoda. 



Tlie body is externally^ and for the most partj 

 internallyj bilaterally symmetrical. It falls naturally 

 into two regions, the head and the body. The „. „„, 



^ . '' Pig. 261. 



head is very vs'ell developed; the mouth is anterior; Teredonavaiis. 



and in all Dibranchiata [i.e., in all Cephalopoda, except " ^^^^^' * *°°*' 



Nautilus) is surrounded by eight long muscular arms.* syphons. 

 In some Dibranchiata, i.e., in the Decapoda, there are 



* In certain of the Cephalopods some, or aU, of the eight arms are connected at their 

 bases, or further up, by a thin web of sMn (just as are the toes of many swimming 

 Mammals and Birds). 



