356 



LOWER INVERTEBRATES. 



project but a short distance from the body. The shell is absent, and the foot is pro- 

 vided only in the male with a sucking disc. Two genera are recognized. In Ptero- 

 trachea (= Firola) no tentacles are present in either sex, and the tail is pinnate. In 

 the other genus, Firoloides, tentacles are present in the male, but the tail fin is simple 

 and slender, and the gills may be present or absent even in the same species. The 

 egg sti'ing of the Pterotracheans is much like that of Carinaria ; it is long and 

 subcylindrical, and the eggs are imbedded in a single row in the glassy matrix. Ovi- 

 position appears to take place the whole year round, but most abundantly between 

 September and March. The Pterotrachem seem to possess great vitality and to with- 

 stand injuries which would be certain death to other forms. The loss of the gills is 

 of no especial consequence, and indeed they are so small in proportion to the body 

 that they are of no great use, and are to be regarded as rudiments of respiratory 

 organs which have been retained during a retrograde development. This vitality 

 goes even farther, as the following quotation will show : — "Atiops peronii, described 

 and figured as having no head, was probably a mutilated Firola. ' Such specimens 

 are very common, and seem just as lively as the rest.' " ♦ 



Sub-Class III. — Pteeopoda. 



The wing-footed MoUusca is another group, the position of which is far from 

 certain. By some it is made a class equivalent to the acephals and cephalopods, 

 some place it near the base of the cephalophorous series, and one 

 recent author has regarded it as a meipaber of the Cephalopoda. The 

 best authorities are, however, inclined for the present to regard it as 

 a member of the Cephalophora, assigning it a position at the top of 

 the group, a course which we will adopt. 



The pteropods are pelagic molluscs which derive their name from 

 the fact that portions of the foot (epipodia) are expanded and fitted 

 for aquatic flight. The body is sometimes long and stretched out in 

 a straight line ; at others it is coiled in a sjsiral. The anterior part, or 

 head, is usually not plainly differentiated from the foot. In some the 

 mantle is not well marked, and the body is naked ; in others the 

 mantle is well developed and secretes a glassy, horny, cartilaginous, 

 or even calcareous shell, the two sides of which are almost invariably 

 alike, and into which the whole body can be retracted. At the an- 

 terior end is the mouth, which is either surrounded by two tentacles, 

 or six protrusible processes armed with minute sucking discs, or by 

 (as in Pneiimodermon) two long ai'ms bearing suckers like those of 

 a cuttle-fish. The mouth is provided with jaws and a lingual ribbon, 

 the teeth of which vary between moderate limits, some having twenty- 

 five, and others as few as three in a transverse row. Salivary glands 

 are present, and just behind the entrance of their ducts the alimentary 

 canal widens into a stomach, and then contracts to form the intestine, 

 which, after a number of convolutions, bends on itself, terminating 

 usually on the right side near the edge of the mantle. 



The circulatory organs are but poorly developed, and a closed sys- 

 tem does not exist. The arteries end with wide openings in the body cavity. Here the 

 blood passes about between the tissues through spaces without proper walls, going to the 



FlG.466.— Oeseis acic- 

 ula; b, brain; h, 

 heart; r, reproduc- 

 tive organ; «, sto- 

 mach; w, wings. 



