CHARACTERS CT THE SUfi-ffflfolfiOllS. 21 



parieto-splanchnic ganglia (Gr. parietes, walls; splanchna, 

 the internal organs). 



Respiration is generally effected by gills or branchiae. 

 In the land snails, and in some fresh water molluscs, 

 the breathing organ is termed a pulmonary sac. It 

 consists of an opening hollowed out on the right side 

 of the body, near the neck, and supplied with blood- 

 vessels. The blood in these animals is purified by coming 

 directly into contact with the oxygen of the atmosphere, 

 and not through the medium of water as in those that 

 breathe by gills. 



18. The sub-kingdom Mollusca is divided into the fol- 

 lowing classes, viz.: 



1. Cephalopoda (Gr. cephale, the head ; poda, feet). 



2. Pteropoda (Gr. pteron, a wing ; poda, feet). 



3. Gasteropoda (Gr. g aster, the stomach). 



4. Lamellrbranchiata (Lat. * lamella, a plate ; Gr. 

 Iranchia, a gill). 



SUB-KINGDOM V. Molluscoida. 



19. This division is allied to the Mollusca, but its 

 characters are sufficiently distinct to entitle it to rank 

 as an independent group. 



The heart, when present, is in the form of a simple tube 

 or sac, and is never separated into auricle and ventricle. 



The nervous system generally consists of a single 

 ganglion. 



The mouth is either surrounded with ciliated tentacles, 

 or with long arms, which have a ciliated fringe. 



The alimentary tube has two openings, and is com- 

 pletely shut off from the cavity of the body. 



There are three classes in this sub-kingdom : 



1. Brachiopoda (Gr. brachia, arms; poda, feet). 



2. Tunicata or Ascidioida (Lat. tunica, a cloak ; Gr. 

 askos, a bottle ; eidos, form). 



3. Polyzoa (Gr. polus, many; zoon, an animal). 



