MOLLTJSCA. 153 



row of cilia ; and (3) the Veliger-stage, when it has acquired the 

 " velum " a ciliary cephalic expansion of the integument, said 

 to be identical with the ciliated disk of the Eotifera. The ova 

 are often defended by hard albuminous capsules of various, and 

 sometimes very complex, forms. 



" In the Mollusca the developmental energies seem to have been 

 expended chiefly in the perfection of the vegetative series of 

 organs, or those concerned in the immediate preservation of the 

 individual and the species." 



The limits of the Mollusca are at present unsettled. Hay 

 Lankester places the Tunicata with the Vertebrata, Hackel and 

 other biologists with the Vermes. Molluscoida included Polyzoa, 

 Brachiopoda, and Tunicata. A " roughly " drawn distinction is 

 sometimes made between those Mollusca with a head [Encephala 

 or Cephalophora] and those without [Acephala]. In the former 

 the head is generally provided with tentacula, eyes, and a mouth 

 armed with jaws ; in the latter there is no cephalic ganglion, and 

 the mouth is a simple inlet for the food, " having no power of 

 selection in the first instance." Otocardia, excluding Brachiopoda 

 and Tunicata, is another name for the Mollusca. Schmarda has 

 seven classes, including Polyzoa ; Glaus four, excluding Brachio- 

 poda and Tunicata, Gastropoda including Heteropoda and Pte- 

 ropoda, while Scaphopoda is raised to the rank of a class. 



Animal enclosed in a bivalve shell. 



Mouth with two arms BRACHIOPODA. 



Mouth without arms LAMELLIBRANCHIATA. 



Animal either naked or enclosed in a uni- 

 valve shell. 



With a head ; heart with two or more 

 chambers. 



Mouth with long arms CEPHALOPODA. 



Mouth without arms. 



Locomotion effected by a ventral 



disk GASTROPODA. 



Locomotion effected by fin- or 



wing-like appendages. 

 With a fin-like tail or a ventral 



fin HETEROPODA. 



With two wing-like expansions... PTEROPODA. 

 Without a head ; heart a simple tube ... TUNICATA. 



