110 



SHELLESS ACEPHALES. 



I have had occasion to write so much, and which bore some 

 in wood and some in rock, as you are well aware. At the 

 end of the family we find a doubtful station assigned to the 

 Aspergillum (Fig. 17), or water-pot shell, of which a perfect 

 specimen will be a very desirable addition to your 

 fig- 17. collection. 



The AcEPHALES SANS CoQuiLLEs, the se- 

 cond order of the class, are so dissimilar to 

 the first, that Cuvier says it might be raised to 

 the rank of a distinct class, if such were thought 

 convenient. Their branchiae assume diverse 

 forms, but are in no instance divided into four 

 leaves or lamellse ; on the contrary, they usually 

 form a regular net-work on the inner surface 

 of the tunic. Shell they have none, and conse- 

 quently the conchologist disallows their claims 

 upon his attention, but there is no order more 

 interesting to the physiologist. The coverino- 

 which stands in lieu of a shell is a coriaceous, 

 or cartilaginous, or^ sometimes, a merely fleshy 

 coat, with two circular apertures on some part 

 of it ; one for the admission of the water con- 

 taining all the necessary food and air, the other 

 for the discharge of their excretions. There are 

 two families. The first comprises the genera, of 

 which the individuals are isolated and com- 

 plete in themselves, though sometimes occur- 

 ring in clustered groups ; the second, those which 

 are compound beings, a certain number of in- 

 dividuals being organically associated to constitute one 

 body. The Salpa? — singular gelatinous animals which float 

 in the sea in long ribbon-like chains or with every link de- 

 tached,— belong to the first, where likewise are located the 

 Ascidies, very unlike the former in all things, in the form 

 and structure of the cloak, in the disposition of the branchiee, 

 and in their permanent fixidity to rocks, shells, or other 

 foreign bodies. The compound family (les Aggreges) is a 

 very remarkable one, connecting the molluscans with the zoo- 

 phytes, among which they were arranged, until the laborious 

 researches of Savigny proved the fallacy of their position : 

 it contains locomotive genera, such as Pyrosoma, one of the 

 most wonderful of created beings ; and genera permanently 

 fixed, such as Polyclinum, with its animalcules disposed in 

 star-like figures shining through a pellucid jelly. 



The Brachiopods have considerable affinity to the bi- 



