GASTEROPODA SCUTIBRANCHIATA. 385 



ORDER VIII. 



SCUTIBRANCHI ATA( 1 ). 



The Scutlbranchiata comprise a certain number of Gastero- 

 poda, similar to the Pectinibranchiata, in the form and position 

 of the branchiae, as well as in the general form of the body, 

 but in which the sexes are united, in such a way, however, as 

 to allow them to fecundate themselves. Their sh-ells are very 

 open, without an operculum, and most of them without the 

 slightest turbination, so that they cover these animals, and 

 particularly their branchiae, in the manner of a shield. The 

 heart is traversed by the rectum, and receives the blood from 

 two auricles, as is the case in the greater number of bivalves. 

 The 



Halyotis, Lin. (2) 



Is the only genus of this order in which the shell is turbinated^ it is 

 distinguished from that kind of shell by the excessive amplitude of 

 the aperture, and the flatness and smallness of the spire, which is 

 seen from within. This form has caused it to be compared to the 

 ear of a quadruped. In the 



Halyotis, Lam., 



Or the true Halyotes, the shell is perforated along the side of the 

 columella by a series of holes; when the last hole is not terminated, 

 it gives to that part the look of an emargination. The animal is one 

 of the most highly ornamented of all the Gasteropoda. A double 

 membrane, cut into leaves and furnished with a double range of 

 filaments, extends, at least in the most common species, round the 

 foot and on to the mouth; outside its long tentacula, are two cylin- 

 drical pedicles which support the eyes. The mantle is deeply cleft 

 on the right side, and the water, which passes through the shell, 

 penetrates through it into the branchial cavity; along its edges we 



(1) M. lie Blainville unites this order and the following one (the Chitones ex- 

 cepted) in his sub-class of the PARAC£flIALOPHORA HERJIAPHnODITA. 



(2) The Paracephaloph. Hermaph. Otid., Blainv. 



Vol. II.— 2 Y 



