140 Selections. [No. 7, new series. 



Sub Class of the Heteropoda. 



This sub -class comprises a few swimming animals with spindle- 

 shaped bodies, which are glass-like and transparent, and some- 

 times possess a pretty little shell, which however contains but a 

 very small part of the body. The swimming apparatus consists 

 of a keel-shaped float or fin, which is turned upwards when the 

 animal swims. The head is distinct, and provided with feelers 

 more or less well formed, and generally possesses two large eyes 

 and a long proboscis, at the fore-end of which the prickly tongue 

 is rolled up. 



We distinguish three families : 



1st. The " Atlantides" (Atlantida) possess a snail-like shell 

 into which the animal can withdraw completely. The head is pro- 

 vided with feelers, eyes and a short proboscis, and is placed on an 

 elongated neck, on the lower side of which are placed the swim- 

 ming flaps provided with a sucking cup ; and a tongue shaped 

 prolongation bearing a thin legument which closes the opening of 

 the shell when the animal is entirely withdrawn. Atlanta. 



2nd. The family of the " Keel-snails" (Firolida) is either en- 

 tirely naked, or has a little cap- shaped shell. The body is some- 

 times as much as a foot long, spindle shaped and so transparent 

 that in a perfect specimen all the peculiarities of the organisation 

 can be clearly traced. These animals appear in swarms on the 

 surface of the Mediterranean but their shell is so brittle that it is 

 seldom obtained perfect. Carinaria, Pirola, Pterotrachea. 



3rd. The family of the " Arrow-snails" (Sagittida) is only 

 placed provisionally in this sub-class : — it consists of little spindle- 

 shaped animals with a distinct head which have hitherto only been 

 found in the Mediterranean and North seas. (See fig. 12, plate 

 vi). At the sides of the body are two pair of horizontal swim- 

 ming flaps, and the tail end is surrounded by a swimming flap, 

 shaped something like the feathered end of an arrow. The mouth 

 is armed with lateral hook-mandibles, and on the head are two 

 slightly developed eyes and feelers. The activity with which this 

 animal moves is remarkable. Sagitta. 



