26-1 



THE POPULAE EDUCATOH. 



attributed to the little winged creature from which they are 

 reflected. 



In the Mollusca instincts are not wanting, and locomotive 

 and perceptive powers are also evidenced ; but they are wholly 

 inferior in these faculties to the Articulates, and in turning to 

 them we feel to be turning from the higher modes of life to the 

 lower. The body of the in- 

 sect is so fitted for rapid and 

 dexterous action that it seems 

 to have become almost ethe- 

 real; while in the slug the 

 very senses seem to be dulled 

 to suit the requirements of the 

 coarser material parts. 



For a further contrast of 

 the Articulata and Mollusca 

 we must refer the reader to a 

 former lesson. It suffices here 

 to mention one or two points 

 of general structure which are 

 intimately connected with the 

 habits of these creatures. 

 First, since the Mollusca are 

 less adapted for locomotion 

 than other animals, the re- 

 quirements which swift loco- 

 motion dictates are not in- 

 sisted on with such strictness. 

 Rapid locomotion necessitates 

 what is called bi-lateral sym- 

 metry that is, that the body 

 shall be precisely alike on both 

 sides. This, accompanied with 

 an elongated form, whose axis 

 is in the line of locomotion, is 

 the form best suited for ad- 

 vance. A ship is better than 

 a tub for locomotion, and the 

 keel must not only be straight, 

 but the hold must not be lop- 

 sided. Thus we found that in 

 the insect every part, down to 

 the shape of the nerves in its 

 netted wings, is accurately re- 

 presented by its counterpart 

 on the other side. In the 

 Mollusca, however, this sym- 

 metry is not essential, and it 

 is liable to be dominated by 

 other requirements. A pecten 

 (scallop) lies on its side, and 

 hence has one side flat and the 

 other rounded. A snail has its 

 body rolled into a spiral for 

 compactness' sake, and for the 

 convenience of having a large 

 orifice for its respiratory cham- 

 : ber ; that spiral is nt equi- 

 lateral, but twisted to one side. 

 Secondly, since rapid locomo- 

 tion is not the first object, the 

 walls of the body represent a 

 rounded bag to contain the 



is directly from the Hydroid Coelenterata. These creatures are 

 compound animals that is, the product of a single egg or repro- 

 duced' animal may grow into a multitude of similar heads or 

 parts, each one of which has all the essential organs of life, and 

 can live when detached ; hence their name Polyzoa, or multiple 

 animals. The plant-like growth of these compound animals is a 



process of budding, but it is 

 carried on in very various 

 directions. According to these 

 directions and methods, very 

 various aggregate forms aro 

 produced. In many instances 

 it results in an incrustation 

 composed of closely-set cells. 

 These crusts spread from a 

 point, and often closely invest 

 sea-weeds and rocks. They 

 seem to have a special par- 

 tiality for the shells of other 

 Mollusca, exercising, however, 

 a discrimination by affixing 

 themselves rather to the shells 

 of the less locomotive bivalve 

 Mollusca than to those of the 

 univalves. Thus they have 

 the benefit of a slight change 

 of place without being subject 



viscera, rather than a fulcrum, 

 upon which lever-like limbs 



are plied; and hence it is al- *0KU.-1. FLUSTRA. 

 ways soft and flexible, and the TO SHOW INTERNAL ORGANS - 

 limbs are not jointed organs, 

 but lobes or protrusions of 



the bag wherewith to crawl or grasp, not to raise or propel. 

 Thirdly, since protection is not sought in flight, it is afforded 

 in a structure which would impede flight, and the creatures, 

 when on the defensive, retract and nullify all their feeble loco- 

 motive apparatus, casting themselves wholly on the defensive 

 efficiency of their shells. 



The lowest class of the Mollnsca is widely different from the 

 more typical classes, and the structure of its representatives 

 seems to indicate that the starting-point of the molluscous classes 



to abrasion by constant loco- 

 motion. These incrustations 

 so much resemble the clothing 

 of moss which spreads over 

 stones and walls, that some 

 naturalists have applied the 

 name of Bryozoa to these 

 animals, a term which signifies 

 moss-like animals. The crusts 

 are to be found by any one 

 who examines objects which 

 are generally covered by tho 

 sea, and, when examined by 

 the naked eye, they look like 

 fine lace of different patterns. 

 So great is the diversity and 

 beauty of these forms that 

 they repay even this superfi- 

 cial investigation. The Poly- 

 zoa have existed from a very 

 early period in the history of 

 the earth, and they then had 

 the same habits, as a class, as 

 they now have. This is evi- 

 dent from the fact that no 

 one can make a collection of 

 fossil shells without also col- 

 lecting many of the Polyzoa 

 attached to them. In these 

 cases they are usually attached 

 to the outside of the shells, 

 When they are found covering 

 the inside of the shell, of 

 course this is a proof that the 

 shells remained for some time 

 at the bottom of the ancient 

 sea after the death of their 



owners, before they were cc- 

 II. PLUMATELLA REPENS (ONE POLYP) CUT OPEN vered ^ ^ mud or gand 



III. SCRUPOCELLARIA, SHOWINQ VIBRA- . ,, ... ,. , ,, 



TM WT innmMU TWT PT.T IT. f the Deposit in which they 



are found embedded. One 

 comparatively recent forma- 

 tion is so full of the remains of these creatures as to be called 

 the Coralline Crag of Suffolk. This name was given when the 

 creatures were supposed to resemble coral polypes (Actinizoa), 

 and now that they have been discovered to be of higher standing- 

 in the animal scale, and, in fact, to belong to an altogether 

 different sub-kingdom, of course the name is inappropriate. 



Sometimes the crusts do not cling closely to the substance 

 from which they spring, but rise up from it. In such cases the 

 free frond may be flattened, being, in reality, two crusts placed 



TILE ORGANS. IV. ARICULARIUM, WITH MUSCLES THAT PLY IT. 



