MOLLUSCA 



281 



and on the siphon. Tentacles, or special tactile organs are common on 

 the siphon and mantle edge. 



609. Fertilization of the eggs takes place in the mantle cavity where the 

 early stages of development also take place in many cases. In the fresh- 

 water clams especially, the _ 



larvae remain for a long time '"^^H^^^^^^^^St^ 

 attached to the gills of the 

 parent. The marine forms 

 have a trochophore larva. In 

 the fresh-water forms the 

 metamorphosis is more com- 

 plete and in some cases the 

 larvae live for a time as para- 

 sites attached to the gills and 

 fins of fishes. 



610. All Lamellibranchs are 

 aquatic and chiefly marine. 

 Most live free on the bottom 

 but some are attached by 

 byssus threads which are 

 formed by the secretion of a 

 byssus gland in the small foot. 

 Others are attached by the 

 cementing of one valve to the 

 substratum. Some bore into 

 wood and others into calcare- 

 ous rocks. 



61 r. Order 4. — The Cepha- 

 lopods are the most highly 

 organized of all molluscs and 

 in some respects of all inver- 

 tebrates. All are marine and 

 some attain great size. Some 

 species are known to attain a 

 length of 50-60 feet including 



the long arms. The squid, cuttlefish, nautilus, and octopus are some 

 of the best-known examples. Except in the pearly nautilus the 

 shell is always rudimentary, and completely overgrown by the mantle. 

 The visceral mass is elongated, conical in form, and lies in a much 

 larger mantle chamber. There are two or four plume-like gills. The 



Fig. 166.— The "soft shell" clam, :Mya 

 arenaria. Showing the position when 

 buried in the mud with the siphons extend- 

 ing to the surface. (From Galloway after 

 Kingsley.) 



