COMPARATIVE ANATOMY. 



Ml 



back to book, and mutually supporting each other. This in the 

 cao with the ordinary sea-mat (Fluatra maxima), which in one 

 of the largest of these compound animals, and may hare been 

 often mistaken for a sea-weod by the casual observer. In other 

 cases the stems, instead of being wide and flat, may be much 

 narrower, presenting single or double chains of cells, which 

 branch and spread freely in the water. In some rarer instances 

 the whole compound animal is locomotive, the crust, with its 

 many colls, and their contained simple zooids, travelling, by 

 common consent, in one direction. It will be seen by the above 

 description that the outward form of these creatures is closely 

 paralleled by those of the Coelenterata, and when to this out- 

 ward resemblance is added the fact that each of the simple 

 animals which occupy the cells has a head almost precisely simi- 

 lar to the Hydro- 



zoa, it is not sur- 

 prising that they 

 should have long 

 been confounded 

 with them. A 

 mouth, surrounded 

 by a circle of ten- 

 tacles capable of 

 motion, is charac- 

 teristic of both 

 the compound Hy- 

 drozoa and the 

 compound Poly- 

 zoa; but it was 

 early observed that 

 the tentacles of 

 the latter were 

 always clothed 

 with cilia, while 

 those of the former 

 were unfurnished 

 with them. The 

 reader of these 

 lessons will re- 

 member the na- 

 ture of the organs 

 called cilia, and the 

 nature of their ac- 

 tion as described 

 when the Rotato- 

 ria wore treated of. 

 By the aid of these 

 organs currents 

 arc set in motion 

 and directed to- 

 wards the funnel- 

 shaped mouth, and 

 thus small parti- 

 cles of food are 

 procured in addi- 

 tion to that which 

 can be obtained by 

 the action of the 

 whole tentacle, or 

 of many of these 

 acting together, when they seize a larger prey in the same way 

 as the Hydrozoa obtain the whole of their food. A difference of 

 this superficial character, founded on organs which clothe the 

 outside of the animals, did not at first seem sufficient to justify 

 any great division being constituted between the Polyzoa and 

 Ccelenterata polypes, and hence they were called ciliated 

 polypes. But it was soon found that this external difference 

 was associated with a fundamental difference in the internal 

 organs. The food collected by the ciliary action is passed not, 

 into the general cavity of the body, to be there retained, and 

 the refuse to be ejected again from the mouth, as in the true 

 polypes, but into a definite stomach, and passed through this 

 along a complete tube, completely shut off from the general 

 cavity of the animal, which contains the nutritive fluid, and is 

 bounded by the hard cell wall. Thus we have, as in all the 

 higher animals, a tube within a tube, and a distinction and 

 division between the raw material of nutriment and the changed 

 and elaborated product of digestion ; and this distinction is 



rightly considered to be of high physiological importance, and 

 quite sufficient to determine that the Polyzoa belong to the 

 great and distinguished sub-kingdom of the Mollnsca, and may 

 from henceforth out their acquaintance, the humbler polypi. 



The cells which compose the outer skeleton of the Polyzoon 

 are, of course, all for the protection of the ""*V Whether 

 they MO tabular, as in the Plumatella, cup or cradle-shaped, M 

 in the Scrnpocellaria, or like poaches, a* in many other forms, 

 they always furnish retreat* into which the whole "<"relfifc, 

 with its delicate tentacles, can be withdrawn. When BO withdrawn, 

 however, the animal has only retired within itself, for the cell, 

 whether composed of chalky, horny, or gelatinous substance, is 

 really the external wall of the creature, strengthened by, or con- 

 sisting of, deposits of their substance*. In order that the 



1L 



be protruded or 

 retracted from the 

 cell at pleasure, it 

 is, of course, ne- 

 cessary that some 

 considerable port 

 of the outer wall 

 which joins it to 

 the hard part of 

 the same should 

 be soft and flexi- 

 ble. A reference 

 to the illustration 

 of Plnmatolla will 

 show the relation 

 of the flexible to 

 the hard part of 

 the wall, and the 

 muscles by which 

 the head is pulled 

 back. This illus- 

 tration will also 

 show the character 

 of the alimentary 

 canal, and how it 

 is bent upon itself, 

 so that the after 

 part passes up 

 close to the first 

 descending por- 

 tion. This ar- 

 rangement of the 

 food canal is very 

 constant in the 

 Polyzoa, and is 

 dictated by the 



TUNICATA. I. PEROPHORA LISTERI. II. SALPA MAXIMA (ONE OF A CHAIN TO SHOW CIRCULATION) . 



III. DIAGRAM OP A SOLITARY TUNICATE. IV. TRANSVERSE SECTION OP SOUTABT TUNICATE. animal can 



Kefs, to Nos. in Figs. I. 1, nervous ganglion; 2, atrial chamber and outlet; 3, respiratory pharynx protrude one end 



with its slits ; 4, stomach ; 5, portion of heart ; 6, growing bud. II. 1, front opening ; 2, hind of its body from 



ditto; 3, 3, places of attachment to chain of salpee; 4, respiratory band; 5, heart; 6, mass of 

 viscera. III. 1, tentacles; 2, pharynx; 3, stomach; 4, anus; 5, oviduct; 6, anal opening; 7, 

 oval opening. IV. 1, test, or tunic ; 2, muscular coat ; 3, third tunic ; 4, the same reflected 

 on the pharynx ; 5, perforated pharynx ; 6, endostyle ; 7, anus. 



the cell, both the 

 entrance and the 

 exit must be at 

 that end. Muscles 

 originating from 



the bottom of the hard part are attached to the stomach and 

 throat, so as to pull these parts back when the head is with- 

 drawn. In the re-entering angle between the month and arms & 

 single ganglion is found, and this sends a few nerves round the 

 throat and to the body walls, in some, at least, of these crea- 

 tures. The reader will have gathered from the illustration and 

 description that while some of these creatures have all the body- 

 cavities of their simple zooids in communication with one 

 another, others have these completely separate. In both cases, 

 however, the simple animals seem, in many respects, dependent 

 in some measure on the general structure. Thus many of them, 

 have external organs not possessed by each cell, but only by 

 some of them, which organs, nevertheless, minister to the wants 

 of all. These external organs are very singular, and are of 

 two kinds. One kind is like a vibrating whip, and the other 

 resembles the head and beak of a bird. Both are endowed with 

 a power of motion which is apparently automatic. It is sup- 

 posed that the whips constantly stir the water, so as to bring 



