360 MANUAL OF THE MOLLUSCA. 



Discmce often have minute fry attaclied to their yalves, and Mr. 

 Suess, of Vienna, has noticed a specimen of the fossil Stringo- 

 cepJialus, which contained numerous embryo shells. 



As yet we know little respecting the development of the 

 SracMopoda, but there can be no doubt that in their first stage 

 they are free and able to swim about until they meet with a 

 suitable position. It is probable that in the second stage they 

 all adhere by a byssus, which in most instances becomes con- 

 solidated, and forms a permanent organ of attachment. Some 

 of the extinct genera (e.g. Bpirifera and StropJwmena) appear 

 to have become free when adult, or to have fixed themselves by 

 some other means. Four genera, belonging to very distinct 

 families, cement themselves to foreign objects by the substance 

 of the ventral valve. 



The nervous system exhibits a state of development but little 

 superior to what is found in Ascidians. No special organs 

 of sense have been detected. The red spots in the mantle, sup- 

 posed by some to be rudimentary eyes and ears, are probably 

 the glands situated at the base of the setae. 



The Lamp-shells are all natives of the sea. They are found 

 hanging from the branches of corals, the under sides of shelving 

 rocks, and the cavities of other shells. Specimens obtained 

 from rocky situations are frequently distorted, and those from 

 stony and gravelly beds, where there is motion in the waters, 

 have the beak worn, the foramen large, and the ornamental 

 sculpturing of the valves less sharply finished. On clay beds, as 

 in the deep clay strata, they are seldom found ; but where the 

 bottom consists of calcareous mud they appear to be very 

 abundant, mooring themselves to every hard substance on the 

 sea-bed, and clustering one upon the other. 



Some of the Brachiopoda appear to attain their full growth in 

 a single season, and all probably live many years after becoming 

 adult. The growth of the valves takes place chiefly at the 

 margin ; adult shells are more globular than the young, and 

 aged specimens still more so. The shell is also thickened by the 

 deposit of internal layers, which sometimes entirely fill the 

 beak, and every portion of the cavity of the interior which is 

 not occupied by the animal, suggesting the notion that the 

 creature must have died from the plethoric exercise of the cal- 

 cifying function, converting its shell into a mausoleum, like 

 many of the ascidian zoophytes. 



The intimate structure of the shell of the BracMopoda has 

 been investigated by Mr. Morris, Professor King, and more 

 recently by Dr. Carpenter; according to this last observer, 



