ZOOLOGY. 



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animal resembles a double-necked bottle, hence 

 the name of Ascidian (ascus, a bag). The inner 

 bag is perforated like the outer. The mouth leads 

 into the large respiratory or pharyngeal sac, 

 which is continued at its lower end into the 

 oesophagus. This res- 

 piratory sac is perforated 

 by a number of aper- 

 d tures, and is richly sup- 

 plied with blood-vessels, 

 for it is here that the 

 blood is aerated. The 

 intestine is twisted on 

 itself, and does not open 

 on the surface directly, 

 but through the medium 

 of a cloacal chamber. 

 Section of Social Ascidian : The circulatory appara- 

 a, mouth; t, vent; c, stomach; tus differs remarkably 



d. intestinal canal; e, common / ii,-* _r _n ~.i,,,, 



tabular stem. from that of a11 other 



animals. It consists of 



a simple open tube without valves, which pul- 

 sates rhythmically. The blood is driven first 

 through one end, and then through the other, 

 so that the course of the blood is periodically 

 reversed. The nervous system consists of a single 

 ganglion placed on one side of the mouth. The 

 Ascidians are all hermaphrodite. Some forms 

 are composite, as Botryllus, which is also fixed. 

 Others, such as Salpa, are free, and float on the 

 surface of the sea, their tunic being so delicate 

 and transparent that they would scarcely be ob- 

 served if it were not for their iridescent appear- 

 ance. The Tiinicata have attracted a consider- 

 able amount of attention lately, from the fact that, 

 while in their embryo state, their nervous system 

 presents appearances somewhat resembling those 

 found in the little fish the Lancelet or Amphi- 

 oxus, the lowest of vertebrated animals. 



CLASS III. BRACHIOPODA. 



The Brachiopoda (arm-footed), are so called 

 because of their possessing two long ciliated arms, 

 supposed to be for creating currents, thus bringing 

 food to the mouth, which is placed at the bases 

 of the arms. The animal is inclosed in a bivalve 

 shell, lined by a mantle. The two valves of the 

 shell are united by a hinge, and kept in position 

 by means of muscles. The valves are generally of 

 unequal size, the larger one being in some cases 

 provided with a beak, which is perforated by an 

 aperture, and hence the common name of Lamp- 

 shells, from their supposed resemblance to the 

 old Roman lamps. The valves with respect to 



Terebratula : 

 a, valve with the spiral arms ; *, valve with arms removed 



the body are placed ventrally and dorsally. The 

 Brachiopoda are all marine, and all the known 



living forms are fixed. The shell is lined by, and 

 is a secretion from the mantle, a strong muscular 

 tunic which invests the soft parts of the animal 

 The long arms are in some genera supported by 

 a calcareous framework called a carriage-spring 

 apparatus. The arms probably perform the func- 

 tion of respiration, and the nervous system con- 

 sists of a single ganglion, placed between the 

 mouth and the anus. It includes the genera 

 Lingula, Terebratula, Crania, &c. The Brachio- 

 poda were very abundant in some rocks, particu- 

 larly in the Silurian formations, where they appear 

 to have attained their maximum of development. 

 The genus Lingula is found in the lowest fossil- 

 iferous rocks, and what is very remarkable, this 

 genus still exists at the present day. 



II. The Mollusca proper, which include the 

 following four classes : 



CLASS IV. LAMELLIBRANCHIATA. 



The Lamellibranchiata (plate-shaped gills) 

 including the so-called ' Shell-fish,' as Oysters, 

 Mussels, Cockles, &c. They have no head, are 

 inclosed in a bivalve shell, and have two lamelliform 

 or plate-like gills on each side of the body, and 

 neatly arranged within the margin of the shell. 

 The body is inclosed in a mantle, from which the 

 shell is secreted. The mantle is more or less com- 

 pletely attached to the shell. This is indicated in the 



m 



Interior of Mussel : 



A, right valve ; B, left valve ; c, hinge ; d, stomach ; e, tentacula ; 

 f foot ; z, byssus ; k, branchial orifice ; /', vent ; k, termination 

 of intestine ; /, liver ; m, gills ; , adductor muscle ; o, ovariura. 



interior of the shell by a line called ^& pallial line. 

 In some bivalves, the edges of the mantle are united, 

 so that a closed respiratory chamber is the result, 

 to which water is admitted by means of a ' siphon/ 

 which is just the mantle lobes united and perfor- 

 ated by two tubes, through one of which the water 

 enters, and is expelled through the other. This 

 siphon can in most cases be retracted within the 

 shell. In the shell of those bivalves which do not 

 possess a siphon, the pallial line is simple, and 

 is not indented ; but in those which have a siphon, 

 the pallial line forms a deflection inwards, like a 

 little bay, to which the muscles that retract the 

 siphon are attached. The valves of the shell, un- 

 like those in the Brachiopoda, are equal and alike, 

 and, with respect to the body of the animal, are 

 placed laterally, that is, right and left. Further, the 

 valves are closed by muscles called adductors, but 

 are opened by an elastic ligament, and a cushion 

 of cartilage, which is placed between the beaks of 

 the valves, so that, when the creature approxi- 

 mates the valves, the elastic ligaments, being 

 placed behind the beaks of the valves, are put on 

 the strain, while the cartilage is compressed, so 



