Ig4 SYNOPSIS OF THE CHARACTERS OF THE 



ACRITA. 



Tlie Acrita form the lowest division of tlie animal kingdom ; they have no appreciable nervous sys- 

 tem no brain, nor nervous filaments, consequently, no organs of sense, and, consequently also, no muscular 

 system ; they have no true circulation, in the highest of them only, has a movement been detected in their 

 nutrient fluids ; and, finally, their digestive system is composed of a few rude cavities in the general paren- 

 chyma of the body, but without distinct walls or abdominal cavity. Of those the Zoophyta are the most inte- 



restintj to the geologist. 



ZOOPHYTA. 



The VMovhyta, or Polyps, exhibit great diversity of structure; the most perfect, or Ciliobrachiate polyps, 

 are very complex, and differ very much from the rest ; they form a separate group, under the name of Bryozoa ; 

 they inhabit tubular cells, and have the tentacula which surround the mouth covered with vibratile cilia, the 

 currents produced by the action of which are sufficiently powerful to hurry the minute animals on which they 

 live into their mouth ; their muscles are fibrous ; the intestinal mass floats in a distinct cavity, and is composed 

 of a mouth, large oesophagus, gizzard, stomach, biliary glands, intestine, and anus ; it is, in fact, as complex as 

 that of a bird, yet some of the most ancient fossil corals we know, apparently belong to this group, as this Flus- 

 tra and Eschara, as well as some others, resembling the ordinary tubular polyps. Of the unciliate polyps, the 

 most hirrhly organized are the tubular polyps, compound animals, whose common body is encased in a branched, 

 tubular, hoi'ny sheath ; the extremities of the branches are open, forming cells, each of which contains a hydra- 

 like portion of the animal, having a mouth, surrounded by ten nodular tentacula, by which food is provided for 

 the entire community. The Actinece, or Sea AnemoTiies, although they could not be found in the fossil state, are 

 interesting to the geologist, from their resemblance to the animal of Turbinolia, Cyathophyllum, and probably 

 Amplexiis. They are short, cylindrical animals, having their base formed into a sucker-like disc, by which they 

 are attached ; the opposite extremity is fringed with numerous, short, perforated tentacula, having the mouth in 

 the centre, which communicates directly with a simple, closed, bag-like stomach ; the space between the sto- 

 mach and the external muscular walls of the animal, contains a great number of vertical, fibrous lamellae; it is 

 those lamellaj, strengthened by calcareous matter, which we see in the fossil Turbinolia, and Cyathophyllum ; in 

 Amplexus, in which the calcareous lamella; are merely rudimentary, they were probably continued in a fibrous or 

 membranous state towards the centre, as in the other species; the spaces between those lamella; contain 

 the ovaries. The Tubiporidce inhabit tubular, calcareous corals ; they live in society, but ai-e not organically 

 connected with each other ; the little animal inhabiting each tube is provided with eight simple, flattened 

 tentacula, each having a few rows of papillje on the edge ; alternating with these are eight slender ovaries, 

 which hang within the tube ; in the centre is a simple stomach, as in Actinia ; the small portion near 

 the mouth of the tube, which is inhabited by the animal, is lined by a membrane, which secretes at the 

 bottom of the cell a transverse, calcareous diaphragm, such as we see in Turbinolia, and Cyathophyllum; 

 those partitions, of course, increase in number with the age of the animal. Next to those we have the 

 Corallidce, which differ entirely from the last, the solid portion of the animal forming a variously branched 

 horny, or calcareous stem, covered by a thick coating of living, soft, irritable matter, in which are seated 

 at intervals the most beautiful and brilliantly coloured polypi, each furnished with eight fimbriated ten- 

 tacula ; those polypi provide nourishment for the entire community ; they are capable of retraction within 

 the cells of the living cortex, but have no immediate connexion with the central axis, in which there are 

 no cells. Then come the Madreporutla, in which there is still an external living crust, and scattered polypi ; 

 but those latter are lodged in cells in the stony, central portion of the animal. The AlcyonidcB scarcely 



