CLASSIFICATION AND DISTRIBUTION OF ANIMALS. 341 



PHYLUM VI. MOLLUSCA. Unsegmented animals possessing 

 a mantle and (usually) a shell. The body is ventrally produced 

 into a muscular locomotor appendage, the foot. The blood- 

 system is largely lacunar, and the coelom is reduced to a peri- 

 cardial cavity communicating with the exterior by two tubular 

 kidneys (? nephridia), one of which is often aborted. Nervous 

 system more or less concentrated, and mainly formed of ganglion- 

 ated cesophageal ring. 



GROUP 1. LAMELLIBRANCHIATA (Anodonta, Unio). 

 Molluscs without a distinct head or an odontophore. 

 Shell bivalve and lateral. Nerve-ring long. 



GROUP 8. ODONTOPHORA (Helix}. Molluscs with 

 odontophore, and a distinct head bearing sense-organs and 

 containing a short nerve-ring. Mantle never bilobed. A 

 univalve shell commonly present. 



PHYLUM VII. CHORD AT A. Segmented animals possessing 

 a notochord and visceral clefts, transitorily or permanently. 

 Central nervous system dorsal and (usually) tubular. (See p. 1 35). 



SUB-PHYLUM L HEMICHORDA (Balanoglossus, Cepha- 

 lodiscus,Rhabdopleura). A small group including three genera 

 of worm-like Chordates possessing a proboscis supported by 

 a smaH notochord. [Rhabdopleura has no visceral clefts.] 



SUB-PHYLUM II. UROCHORDA (Ascidians).A. group 

 of degenerate Chordates in which the larva possesses a tail 

 supported by a notochord, both, however, disappearing as a 

 rule before the adult stage is reached. 



SUB-PHYLUM III. CEPHALOCHORDA (Amphioms). 

 A Chordate group containing a single genus, and charac- 

 terized by asymmetry, very complete segmentation, a noto- 

 chord extending from end to end. and the presence of an 

 atrial cavity. 



SUB-PHYLUM IV. FERTEBRATA.ChoTdates with 

 never more than two pairs of limbs, and usually possessing 

 jaws that move up and down and are not constituted by 

 modified appendages. Skin consists of a several-layered 

 epidermis and a connective tissue dermis traversed by vessels, 



