EXPLANATION OF TEBMS USED. Xlll 



The LIGAMENT consists of two parts, the ligament proper and the CARTILAGE ; 

 they exist either combined or distinct, and sometimes one is developed and not the 

 other. The external ligament is horny, and usually attached to ridges on the 

 posterior hinge margin. The internal ligament or cartilage is generally lodged in 

 pits along the hinge line. The central teeth of the hinge, just under the umbo, are 

 called hinge or CARDINAL TEETH, those on each side LATERAL TEETH. 



The body is surrounded by an integument called the MANTLE, which secretes the 

 shell. In some species the margins of the mantle are united, and there are two 

 holes for the inhaling and expulsion of water ; the margins of these holes are often 

 produced into long tubes or SIPHONS, which can usually be partly or completely 

 retracted within the shell. These siphons are more especially characteristic of those 

 shells that live buried in sand and protrude these tubes in order to obtain nourish- 

 ment. 



Brachiopoda (fig. 9). 



The body of the Brachiopoda is also protected by a bivalve shell, but in this case 

 the two valves are applied to the dorsal and ventral surfaces of the animal. The 

 ventral valve is the larger and has a prominent beak, which is generally perforated 

 by a FORAMEN (fig. 9) for the passage of the peduncle by which the animal attaches 

 itself to submarine objects ; the dorsal or smaller valve is always free and imper- 

 forate. The mouth is furnished with two folded or spiral appendages, united by a 

 membrane, and supported by an internal shelly skeleton (LOOP) on the dorsal valve. 

 On the lower part of the beak of the ventral valve, just below the foramen, there is 

 often a triangular space called the DELTIDIUM (fig. 96). 



The Brachiopoda are all inhabitants of the sea, and are often found in very deep 

 water ; they are always attached to stones, corals, &c., generally by the peduncle, 

 but in the case of the Craniadse, by their flattened lower valves. They are found in 

 all latitudes. 



Polyzoa (figs, i, 2, 3, 4, and 6). 



The Polyzoa always live in associated colonies made up of numerous CELLS, each 

 inhabited by a separate zooid or POLYPIDE. This compound structure, which is 

 called the POLYZOARIUM, is sometimes flexible, and either foliaceous or branching, 

 when they are often mistaken for Hydrozoa ; sometimes it is rigid, and either forms 

 crusts over dead shells, stones, &c., or else assumes an erect form, which may be 

 either foliaceous or branched, and in the latter case they are often mistaken for 

 corals. The Polyzoa can, however, be distinguished from the Hydrozoa by the fact 

 that in the former each polypide inhabits a separate cell that does not communicate 

 with the others, while the latter are united by an interior organic connection ; and 

 from all known recent species of New Zealand corals they can be distinguished by the 

 absence of the radiate septa which characterize that class. 



In the sub-order Chielostomata, the orifice or mouth of the cell is often fitted with 

 a semicircular lip or OPERCULTTM (fig. 2) for closing it. This sub-order is also dis- 

 tinguished by the position and form of an organ termed the OVICELL, and by the 

 occasional possession of other external organs called AVICULARIA and VIBHACULA. 

 The OVICELL (fig. 6b) is generally in the form of a hood above or behind the cell. 

 AVICULARIA may be of three kinds, the SESSILE (fig. 4), the IMMERSED (fig. 6a), and 

 the PEDUNCULATE (fig. 3) ; but whatever its diversity of form, an avicularium always 

 consists of two parts, viz., a moveable mandible and a cup furnished with a horny 

 beak. YIBRACULA consist of a more or less elongated bristle, capable of being moved 

 backwards and forwards. 



The Polyzoa are found attached to seaweed, shells, &c. ; most of them are 

 inhabitants of the deep sea. A few species, however, live in fresh water lakes, or 

 slowly running streams, attached to the under surfaces of stones, leaves, &c., or more 

 rarely creeping slowly over the stems of aquatic plants. 



Tunicata (fig. 5). 



The Tunicata have no calcareous shell, but are enclosed in a tough leathery 

 tunic. They are hollow and elastic, and have two orifices from which they squirt 



