Tunicata. | LOWER PALAZOZOIC MOLLUSCA. 185 
Sect. VIL—MOLLUSCA OF LOWER PALAOZOIC ROCKS (Cambrian and Silurian). 
3rd Sub-kingdom. MOLLUSCA. 
= Heterogangliata (Owen.) 
Turse are all soft, unarticulated animals, often unsymmetrical, and characterised by an unsymmetrical 
disposition of the nervous centres; one principal nervous mass forms a ring round the cesophagus, from 
which various nerves radiate; a pair of ganglia above and at the sides of this collar represent the brain, 
and are variously connected with other ganglia unsymmetrically placed through the body, mostly below the 
alimentary cavity. The sexes are in distinct individuals or hermaphrodite and requiring a reciprocal coitus ; 
oviparous or viviparous. They are either terrestrial and breathing air by pulmonary cavities (as the snails, 
&c.), or inhabit fresh or salt water, and breathe by gills (as all bivalves, and most of the other groups). 
They either swim by fins (as the cuttlefishes), crawl on a ventral disk (as most univalves), float on the surface 
of the sea (as the Janthinw), or are permanently fixed to foreign bodies (as the oysters, W&e,). 
This sub-kingdom is divided into six classes :—1, Tunicata ; 2, Palliobranchiata; 3, Lamellibranchiata ; 
4, Pteropoda; 5, Gasteropoda; 6, Cephalopoda. 
Ist Class.) TUNICATA. 
This class has not been distinctly recognised in the fossil state. It is composed of the lowest organised 
Mollusca having the body enclosed in a tough coriaceous bag, generally fixed to foreign bodies, sometimes 
grouped in clusters; each individual having two small projecting external openings; an inner muscular coat 
adheres to the outer one only at the openings, one of which leads to a large pharynx, covered with cilize for 
respiration, and transversely and vertically plicated, at the bottom of which is a small stomach and a sig- 
moidal intestine, ending near the lateral opening: a constant current of water flows in at the oral and out 
at the lateral aperture. They have a heart and blood-vessels, some of the latter forming part of the stem 
of the compound species; the blood is supposed to ebb and flow; a dendritic ovary occupies the concavity 
of the intestine, and sends a simple duct to terminate near the anus. The young swim by a long tail, and 
resemble some polypes (#ryoz0a) in budding and general structure. 
2nd Class. PALLIOBRANCHIATA. 
Respiration (as the name suggests) by the surface of the mantle, which is of two open lobes, fringed 
and covered with vibratile cilixe; enclosed in a bivalve shell, usually inequivalve and equilateral, one valve on 
the anterior and the other on the posterior aspect of the animal (being, therefore, at right angles to the 
position of the valves of lamellibranchs). There is no head, eyes, tongue, organs of sense, nor mastication. 
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