MOLLUSCA. 55 
Ascidia, and Pyrosoma. Salpa (pl. 77, fig. 3) torms 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 
ageregated individuals produce young which are free, and which are unlike 
the parent; and these free individuals produce a brood of aggregated 
individuals, thus presenting an example of alternate generation. This 
discovery was published by Chamisso in 1830. These animals abound in 
the warin seas, swimming at various depths, commonly with 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 Lect. Embryol., p. 91.) Pyrosoma 
includes compound, gelatinous, and nearly transparent animals, remarkable 
for their brilliant phosphorescence. 
Orver 3. Bracutoropa. “The Brachiopoda ought to be combined with 
Lamellibranchia, having the same structure, and differing only in secondary 
modifications.” (Agassiz.) This order includes bivalve mollusca, 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withdrawn into the shel], and when large it is rolled into a spiral.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 the name Palliobranchiata, 
given to them by Blainville. 
The shell is often adhering, either directly, as in Crania (pl. 76, fig. 24), 
or by means of muscles, as in Zerebratula (fig. 28), 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, 7. caput- 
serpentis, Linn., and 7. psittacea, Gmelin, are found upon both sides of the 
North Atlantic. (See Gould, Znvertebrata of Massachusetts, pp. 141, 142.) 
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 (Moules de Mollusques, 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 difficult 
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 live 
buried in the sand. 
Orper 4. Concutrera. This order contains most of the bivalve shells, 
259 
