266 



THE POPULAE EDUCATOE. 



fresh food within the action of the cilia, while the singular 

 beaka snap up, kill, and hold any small creature till it decays, 

 and so, by disintegration, furnishes food. 



We have indicated the reasons for considering the Polyzoa 

 superior to the Polypi, and before quitting them we ought to 

 state in what respects they are inferior to the other Mollusca, 

 so as to justify us in placing them lowest in the list. Their 

 inferiority mainly consists in the want of a heart, and any true 

 blood-vascular system, and also in not possessing a well-deve- 

 loped liver, which organs are constantly possessed by the other 

 molluscs in great perfection. The liver in these creatures is 

 only represented by some scattered brown-coloured glands 

 placed on the outside of the stomach. 



The Tunicata seem to be more nearly allied to the Polyzoa 

 than the Brachiopoda, and therefore we treat of them next; 

 although it seems to be also true that they are more closely re- 

 lated to the Conchifera than the Brachiopoda are. Perhaps the 

 distinctness of the Brachiopoda as a class may be accounted for 

 by the fact that they are a very ancient class of animals, having 

 inhabited the earth from the earliest primary period. Many of 

 the Tunicata, like the Polyzoa, are compound animals. They 

 are, however, not exclusively so, and they are usually of a much 

 larger size. They do not deposit any chalk or shell in their 

 outer mantle. They have a heart with a single chamber, and 

 a singular respiratory apparatus. 



We will commence by the description of a- simple animal, 

 such as may be found adhering to the under side of a rock near 

 low- water mark. The outer form is that of an Eastern leathern 

 bottle, with a prominent mouth at one end ; but it is unlike 

 this bottle in that it has another opening on the side a little 

 below the mouth. The mouth leads into a wide, dilated throat, 

 which occupies a large part of the centre of the bag-like animal, 

 and is of a very peculiar structure. It has in its walls a series 

 of slits which lead into a cavity directly communicating with 

 the lower orifice. The slits are very numerous and small, and 

 their edges are fringed with cilia which continually drive the 

 water from the central throat to the atrial chamber, as the 

 cavity is called. At the top of the throat is a circlet of ten- 

 tacles which protrude into it, while at the bottom is an opening 

 leading by a short oesophagus into a stomach, from which an 

 intestine, twisted a few times, leads into the atrial chamber 

 near its orifice. The remainder of the viscera, consisting of 

 liver, heart, etc., are closely united with the stomach at the 

 bottom part of the sac. From one end of the heart a vessel 

 runs off, and is continued as a capacious channel along one side 

 of the sac-like throat, while another channel along the opposite 

 side runs to the viscera, and through them to the other end of 

 the heart. The two channels above-mentioned communicate 

 with one another by many transverse vessels, which branch in 

 the membranous walls of the sac. Let us now consider the 

 relation of these organs to the functions of respiration and 

 alimentation. First, with regard to respiration. The cilia 

 which fringe the slits are the prime movers of the water by 

 driving it from the inner sac into the chamber which surrounds 

 it. This motion necessitates that a current should set in at 

 the mouth, or end opening, and another out at the anal or side 

 opening, and thus fresh water is constantly brought to the 

 inner, or what may now be called the respiratory sac. In the 

 substance of this sac the blood is constantly changed by the 

 motive power of the heart, which, though a simple tubular 

 organ, contracts at one end first, and by the successive contrac- 

 tion of its circular fibres drives the blood to one of the channels, 

 and from thence through the tissue of the sac into the other, 

 and so, by way of the viscera, into its other end. It is peculiar 

 to these animals that the current is occasionally reversed. After 

 working in one direction for some time the heart remains at 

 rest, and then begins to propel the blood in the other direction. 

 Next, in relation to food, it will be seen that the current will 

 bring with it many of those little creatures which are so nume- 

 rous in the waters of the sea. By some mysterious action the 

 cilia do not allow these particles of food to pass out by the 

 slits, but propel them down towards the entrance to the stomach, 

 and so onward. The excrement is, of course, passed out at the 

 anal opening by the current. When any obnoxious substance 

 is introduced by the current of water, it is felt by the tentacles 

 which hang down inside the throat, and when this is the case 

 the animal contracts the outer muscular coat so forcibly as to 

 compress not only the atrial chamber, but also the respiratory 



sac, and so drives the water and the substance out again. 

 This action has procured for the simple tunicate the name of the 

 Sea Squirt. 



Compound Tunicates do not differ much in structure from 

 the simple ones, but they are connected together in some in- 

 stances by a stalk, through which the blood is driven from zooid 

 to zooid. This is the case in the family of the Clavellinidae. In 

 the family called BotryllidaB the separate zooids are only the 

 products of a budding process, and though they are at first con- 

 nected organically and always cohere, yet when mature the 

 vascular connection is obliterated, and each feeds and respires 

 for itself. In another family a multitude of zooids are united 

 in a tube, one end of which is closed and the other open. All 

 the mouths, or in-current orifices, of these are outside the tube, 

 and all the ex-current orifices inside, so that the current of water 

 which passes into the tube, being compelled to pass out at one 

 end, drives the whole animal along ; these creatures differing 

 from the foregoing families in being free and locomotive. 

 Another free and locomotive family is characterised by what is 

 called an alternation of generations. In these a solitary indivi- 

 dual gives birth by budding to a whole chain of zooids unlike 

 itself, and united to one another end to end, not, indeed, organi- 

 cally, but by simple attachment. These have their in-current 

 orifices at one end, with a valve attached to them, so as to pre- 

 vent the water escaping outward. When, therefore, the body 

 is contracted the water is driven out at the other end, and so 

 contributes to the onward motion of the chain. Across the 

 respiratory sac there is a band or ribbon stretched, and this is 

 the main instrument of respiration. One of these creatures 

 (i.e., one link of the chain) is represented in the engraving. 

 Each zooid, or link, gives birth to one solitary form, unlike itself 

 but like its mother, and so the so-called alternation of genera- 

 tions is completed. The production of the solitary Salpa is a 

 true reproductive process corresponding to the rearing of a 

 plant from seed, but the production of the chain is analogous to 

 the growth of a branch from a leaf -bud. 



LESSONS IN GEOGRAPHY. XXXII. 



CONSTRUCTION OP PROJECTION OF MAP OP ASIA. 



THE method to be adopted by the learner in preparing a pro- 

 jection for a map of Asia is precisely the same in every respect 

 as that laid down in Lessons in Geography, XXV. (Vol. II., 

 page 355), with the exception of the position of the point from 

 which the arcs representing the parallels of latitude are de- 

 scribed ; which, however, is determined in a similar manner to 

 that adopted for finding the centre from which to describe the 

 arcs representing the parallels of latitude of a map of Europe. 

 A base line must first be drawn, as c D in Fig. 17 (Vol. II., 

 page 356). This line must be bisected in E, and through E a 

 straight line A B must be drawn, which will represent the central 

 meridian line of the map. It will be convenient to number this 

 line as the 85th meridian east from Greenwich. Now take any 

 space to represent ten degrees, but be careful that the space 

 thus assumed is not taken too long in proportion to the base 

 line of the map, and set off from E, the point of intersection 

 of the base line c D, and the perpendicular A B, fifteen of these 

 spaces, and number the points, beginning with E, 5, 10, 15. 20, 

 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, and through the 

 point numbered 80 draw the straight line o H parallel to 9 D. 

 The figure of a projection of a map of Asia, it should be said, 

 is not given here, as the method of constructing it will be made 

 sufficiently clear to the learner by directing his attention to 

 the lettering of Fig. 17, to which reference has been already 

 made above. Now, as in the projection of the map of Europe, 

 the straight line G H will be the limit of the map towards the 

 top, and the points numbered upwards in succession from E will 

 be the points through which will pass the parallels of latitude 

 corresponding with the numbers. To find the centre from which 

 to draw the arcs representing the parallels of latitude, set off 

 five spaces more, of five degrees each, and bisect the last space 

 thus set off. The point of bisection of the last space thus 

 obtained will be distant 12 30' from the point representing the 

 pole, and which would be numbered 90, continuing the number- 

 ing from 80 upwards. This will determine the point from which 

 the arcs representing the parallel of latitude may be described 

 through the points numbered 5, 10, 15, 20, etc., sufficiently near 



