INTRODUCTION TO THE 1NVERTEBUATA. 151 



purposes of digestion and of respiration. The heart proper does not exist, but there are large blood- 

 vessels which are contractile and move the blood. It is the simplest animal amongst the Yertebrata. 



All the animals which are about to be described are invertebrate^ that is to say, they have no 

 jointed, bony, or cartilaginous spinal column, with a brain-case, and limbs, whose bones are connected 

 with the internal skeleton. Even the cartilaginous rod or notochord is not found in any of the adults. 

 This absence of the support and case of the great nervous centres is the great distinction between the 

 animals which may be roughly exemplified by the Cuttle-fish, the Oyster, the Ascidiaii, the Insect, the 

 Worm, the Starfish, the Coral, and the Amcaba ; and the Beasts, Birds, Keptiles, Amphibians, and 

 Fishes. It is a negative distinction, but it is, nevertheless, pregnant with interest, for it seems to 

 establish one of the few great breaks in the continuity of nature. So far as adult and fully-grown 

 forms are concerned, the break is perfect. But were the imperfect young (the embryos) of one group 

 of the Invertebrata the Tunicata to be considered especially, it would be found that they have what 

 may be called the rudiments of a notochord, but placed far back, however, in relation to a tail, and not 

 having the relative position to the nervous system and vegetative organs which is noticed in the 

 Vertebrata. It is also true that there is a great similarity between the minute structures of some 

 Invertebrata and those of the higher animals. 



The Inverfcebrata present almost every variety of shape. Some are without any definite form, and 

 change their shape constantly ; others have no distinct head ; many have the body arranged in joints 

 or segments, and one side of their body resembles the other, and their symmetry is then said to be 

 bilateral. A great group have their structures arranged in a radiate manner like the Starfish, and 

 many others have their head distinguishable from the body. Although no internal skeleton exists 

 with its limbs, after the fashion of the Vertebrata, yet the body may have particles of carbonate of 

 lime or silica here and there in it, and often arranged in beautiful geometric patterns : or a test 

 or shell may cover part or the whole body, or be included within it as a kind of support. In many 

 great groups an external armour of shell or of hard skin is perfect and very elaborate in its varieties 

 of shape and ornamentation. Many have soft skins. Their methods of locomotion are sometimes in 

 relation with the external hard structures, which consist of skin or dermal structures, more or less 

 modified, and provided with mineral matter or a substance called chitine, but this is not always the case. 

 Some move on the ground with a slimy kind of foot, like the snail and slug ; the one is provided with 

 a complete shell into which it can withdraw, and the other has but a small hard portion. Others crawl 

 under stones, make their way in the earth, and move over the surface like worms, and have either a 

 hard coat to their segments, or a perfectly soft and slimy one. Those which lead the life of the insect 

 may have hard or soft bodies at some period of their lives, and may be provided with limbs of more 

 than two pairs in number. In some the development of legs and limbs is so great that nearly every 

 segment of the body has them on opposite sides, and the piercing, sucking, capturing, and masticating 

 organs are really modified limbs, and are not like the jaws of the Vertebrata. Some move by articu- 

 lated limbs like the Crab and the Beetle, others have a coronet of long fleshy tentacles covered with 

 suckers around the head ; wings may exist as in the Butterfly, and correspondingly useful expansions 

 exist around the neck of the swimming Pteropods. But these have no internal skeleton, and the 

 limbs, &c., are essentially skin structures. 



A vast number of the Invertebrata have the body covered with minute and rapidly vibrating 

 structures, visible only under the microscope, called cilia; some are long and others are short, and they 

 move the body in the water with great velocity. Again, some groups of this great division are 

 without these remarkable simple structures. There are members of this lower group of the animal 

 kingdom which move by taking in water and ejecting it in an opposite direction as they swim in the 

 water ; and not a few live in the water, fresh or salt at one, and in the air at another, period of their 

 lives, or air and water are a common home; many live on the surface, and others on the floor of the deep 

 sea. A great number of kinds lead an independent and moving life in their early days, and then fix 

 on to some substance, or on another and larger animal, and remain sedentary, and some of them are then 

 absolutely fixed like the reef-building Corals and the Barnacles ; some may be fixed or remain still by 

 their weight like the Oyster, and others anchor by a set of threads like the Mussel. On the 

 other hand, some kinds, such as the Jelly-fish, are free movers in the water at first, then they 

 .settle down and become fixed and grow unlike their parent, and finally develop young, which, as they 



