SUBJECTS OP NATURAL HISTORY. 195 



The Acalcpha are, for the most part, gelatinous animals, inhabit- 

 ing all parts of the ocean. They are generally transparent, and of 

 a very simple structure ; for instance, the Medusa and Phi/salia. 

 They all excite a tingling or stinging sensation when they touch the 

 skin of man : hence the name of the family. Among the Acalepha 

 are many animals of great size, swimming freely through the sea 

 by their own exertions, possessing a coimplex digestive apparatus, 

 and having, for the most part, numerous long and exquisitely sensi- 

 tive tentacula. There are others, as the Aclin'iee, which are fixed. 



The Ecfiinoderma are also marine animals. The Asterias {^tax- 

 fish). Echinus (Sea-egg), and Cidaris are types of this curious family, 

 specimens of which are before you, many of them with their natural 

 spiny covering. The large fossil tribe Crinoidea (Encrinites, Penta- 

 crinites, and others), which you may study hereafter in the muse- 

 um, is composed of fixed individuals belonging to this family. 



With regard to the anatomical structure of these simple Radiate 

 animals, we find a homogeneous more or less transparent body; 

 sometimes, as in Polygastrica, divided into distinct little sacs or sto- 

 machs ; sometimes (in Poriphera) supported upon a flexible elastic 

 sponge ; at others (Polypiphera) upon a hard calcareous skeleton : 

 but we find nothing like nerves — the peculiar characteristics of a 

 higher order of development — in any of these animals. Nerves first 

 begin to appear in Acalepha : and in the Echinoderma we observe 

 a distinct nervous filament, and the first appearance of muscular fi- 

 bre. 



Frequently found in the interior of the best-protected organs of 

 the higher animals — in the liver, the muscles, the alimentary canal, 

 and even in the substance of the brain — are various species of para- 

 sitic worms, belonging to the class Entozoa. Upwards of fifteen 

 distinct kinds are known to infest the human body, where many of 

 them give rise to well-marked symptoms of disorder — for instance, 

 the Ascarides, Tape and Guinea Worms. A very minute Entozoa, 

 enveloped in a cyst, has been lately discovered in the dissecting-room 

 of St. Bartholomew's Hospital, existing in astonishing numbers, 

 imbedded among the fibres of the voluntary muscles of the human 

 body. They appear to breed and exist in the living flesh, without 

 giving rise to any symptoms yet known. Classed with these Ento- 

 zoa is a singular, long, hair-like animal, inhabiting ditches, called 

 the Gordius aqualicus, from the complicated knots they are capable 

 of forming with their long slender body. I notice it particularly 

 because many persons, not versed in Natural History, think it is 

 merely a horsc-hair animated by being steeped in water ; and some 



VOL. VII., NO. XXII. cc 



