490 THE ANIMAL KINGDOM. 



many marine forms, e. g. Ostrea, and the velum itself is in some fresh-water 

 forms reduced (Anodon, Unto, Cyclas} or absent (Pisidium}. In the case 

 of Cyclas and Pisidium development takes place in a special brood-pouch 

 which is connected with the root of the inner gill-lamella on each side. 

 The young bivalve appears to be nourished by a clear fluid secreted by 

 the walls of this pouch, as are the young of Anodon (and Uniof) by a 

 secretion from the gills. 



P kolas is phosphorescent. The luminous matter is secreted by cells 

 disposed in a band along the anterior edge of the mantle, in two spots 

 at the base of the inhalent siphon and two bands along the same tube. 



The Lamellibranchiata are mostly marine. They are either fixed or 

 free and they occur associated in numbers. They feed on minute organisms 

 suspended in the currents of water caused by the motion of the cilia, 

 clothing the mantle surfaces, the edges of the gill-lamellae, and the labial 

 tentacles. The living genera Area and Mytilus appear in the lower Silurian, 

 and there are large numbers of fossil forms known. 



The class may be divided as follows : 



1. Isomya. Anterior and posterior adductor muscles of approximately equal 

 size. 



(i) Integripallia. Marginal attachment of the mantle to the shell not inflected 

 to form a sinus ; siphons not developed in some, present in most, e. g. Arcacea, 

 Trigoniacea, Unionacea. 



(ii) Sinupattia. Marginal attachment of the mantle to the shell inflected 

 so as to form a sinus into which the pallial siphons can be withdrawn ; siphons 

 always present and large : e. g. Myacea, Pholadacea. 



2. Heteromya. Anterior adductor (pallial adductor) much smaller than the 

 posterior adductor (pedal adductor) ; siphons rarely present : e. g. Mytilacea. 



3. Monomya. Anterior adductor absent in the adult; siphons never de- 

 veloped : e. g. Ostreacea. 



For literature, see pp. 127, 131, 133, 138. 



Anomia, de Lacaze Duthiers, A. Sc. N. (4), ii. 1854. Aspergillum, Id. A. Z. 

 Expt. (2), i. 1883. Mytilus, Sabatier, A. Sc. N. (6), v. 1877. 



Eye, Patten, Mitth. Zool. Stat. Naples, vi. 1886, with lit. quoted. 



PHYLUM ARTHROPODA. 



Coelomate Metazoa, with a bilaterally symmetrical body, composed of a 

 series of somites, usually disposed in dissimilar groups ; with a pair of 

 hollow -jointed limbs attached to more or fewer of the somites ; with a 

 chitinoid cuticle, and body-muscles not arranged in continuous layers, and a 

 nervous system composed of a supra-oesophageal ganglion and a ventral chain 

 of ganglia. 



The segmentation of the body is rarely lost. Successive somites are 



