MOLLUSCA. 55 



Ascldia, and Pyrosoma. Salpa {pi. 77, Jig. 3) forms long lines of aggre- 

 gated individuals. These animals were first distinctly noticed in 1756, and 

 named Thalia in Brown's History of Jamaica, and in 1775 Forskal applied 

 the name of Salpa. The mantle is transparent, and is provided with 

 tubercular suckers, by means of which the bodies adhere together. The 

 aggregated individuals produce young which are free, and which are nnlike 

 the parent ; and these free indivichials produce a brood of aggregated 

 individuals, thus presenting an example of alternate generation. This 

 discovery was published by Chamisso in 1S30. These animals abound in 

 the warm seas, swimming at various depths, commonly witli the back 

 downwards, and coming to the surface in calm weather. Their power of 

 locomotion is slight, the only means being the ejection of water which has 

 been used in respiration. (See Agassiz's Zect. Enibryol..^ p. 91.) Pyrosoma 

 includes compound, gelatinous, and nearly transparent animals, remarkable 

 for their brilliant phosphorescence. 



Okdek 3. Brachiopoda. " The Brachiopoda ought to be combined with 

 Lamellibranchia, having the same structure, and differing only in secondary 

 modifications." (Agassiz.) This order includes bivalve moUusca, named from 

 having a long, spiral, fleshy, arm-like, fringed organ on each side of the 

 mouth, used in securing food, and generally capable of being extended and 

 withdrawn into the shell, and when large it is rolled into a spiral. In the 

 genus Spirifer, which is scarcely more than a Productus, these organs are 

 preserved in a mineralized state. The mouth is provided with fleshy lips, 

 and is situated between the arms. The mantle itself subserves the purposes 

 of respiration, a peculiarity which is recorded in tlie name Palliobranchiata, 

 given to them by Blainville. 



The shell is often adhering, either directly, as in Crania {j?!. ^G,fig. 24), 

 or by means of muscles, as in Terebratula {fig. 23), in which the muscles 

 pass through a perforation in the beak of the left valve. This genus 

 has a curious framework within the shell. The species figured, T. caput- 

 serjyentis^ Linn., and T. imttacca^ Gmelin, are found upon both sides of the 

 North Atlantic. (See Gould, Invertehrata of Massachusetts., pp. 1-41, 112.) 



It has been maintained that in this order, the valves, instead of being 

 right and left, are dorsal and ventral, the large or perforated one being 

 dorsal. Professor Agassiz {Monies dt3 MoJlusgues, -p. 14:) thinks this view 

 is founded upon a false interpretation of the anatomy, and that the 

 Brachiopoda do not present this anomaly. He considers the larger valve 

 as that of the left side, as in Ostrea and Anomia, but whilst the left valve 

 is perforated in Terebratyla, Anomia has the perforation in the right. The 

 descriptions of some conchologists being founded upon the opinion that 

 there is an absence of orientation in the valves of this order, must be read 

 with care or they will not be understood. 



In the genus Lingula the two valves are so much alike that it is difiicult 

 to decide upon their mutual relations. In this genus there is a long pedicle, 

 but probably not for attachment to extraneous bodies, as the species^j|ve 

 buried in the sand. 



OiJDER 4. CoNCHiFEKA. This Order contains most of the bivalve shells, 

 • 259 



