INTRODUCTION. XXV 



liydroids none has been fouml, so that the majority, if not all, of the polyps per- 

 form their complicated movements, ca))tunng and taking in food, digesting it, and 

 reproducing their kind, without the aid of what seems, when we study vertebrates 

 alone, as the most important and fundamental system of organs in the body. 



The Protozoa, sponges, and many ccelenterates depend, for the power of motion, 

 on the irritability and contractility of the protoplasm of the body, whether or not 

 separated into muscular tissue. Referring to the complicated movements of the Pro- 

 tozoa, Dr. Krukenberg well says: "The changeful phenomena of life, which we 

 remark in the smallest organisms — in the rhythm of their ciliary motions, now 

 strengthened, now slackened; in the rhythmic alternation of the capacity of their 

 contractile vesicles; in their regulated incomes, deposits, and expenditure; in the 

 abundance of the visible products of their diverse material exchanges — enable us but 

 remotely to foresee what is here effected by a harmonious co-operation of countless 

 processes limited to the smallest space. Let their formal diiferentiation seem to us 

 ever so slight, just so do these beings become for us all the greater riddles, especially 

 when we find in them vital manifestations elsewhere displayed in tlie living world 

 only by apparatus of the most highly comijlex constructions, and in them meet with 

 processes which, without the orderly co-operation of very different factors, must 

 remain to us unintelligible." In the Hydra for the first time appear the traces of a 

 nervous tissue in the so-called neuro-muscular cells, one portion of a cell being muscu- 

 lar, the other nervous in its functions. 



A more definite nervous organization has been detected in the Actinia;, in the 

 form of disconnected bodies and rod-like nerve cells, and other nervous bodies found 

 near the eye-s])Ots, and the nerve-cells and fibres at the base of the body ; but a 

 genuine nervous system for the first time appears in certain naked-eyed jelly-fishes, in 

 which it is circular, sharing the radiated dispasition of parts in these animals. As the 

 results of his experiments on the ctenophores, Krukenberg finds that animals of this 

 class, of comparatively simple structure, and therefore exhiViiting morphological differ- 

 ences which to us seem trifling, may nevertheless display very diverse reactions when 

 exposed to similar abnormal conditions in the physiological laboratory. "In our 

 attempt to explain the occult vital jiowers thus revealed, we are debarred from an 

 aj)peal to the apparently corresponding diversities sometimes encountered in the case 

 of the much more comj^lex vertebrates." The echinoderms have a well-developed 

 nervous system, consisting of a ring (without, however, definite ganglia, though 

 masses of ganglionic cells are situated in the larger nerves), surrounding the oesopha- 

 gus, and sending a nerve into each arm ; or, in the holothurians, situated under the 

 longitudinal muscles radiating from that muscle closing the mouth. Recent researches 

 on the star-fish sliow, however, that liesides the ring around the mouth, and the five 

 main nerves passing along the arms or rays, there is a thin nerve-sheath which encloses 

 the whole body, and is directly continuous with the external epidermis, of which it 

 forms the deepest Layer. The circumoral and radial nerves are believed to be simply 

 thickenings of this thin nervous sheet. 



In this connection should be mentioned the exijcriments made l)y Romanes, Ewart, 

 and Marshall, on living Echini, " which lead them to believe in the existence not only 

 of an external nerve-plexus outside the test, but also of an internal plexus on its inner 

 surface ; they further believe that the two systems are connected liy nerve-fibi-es run- 

 ning through the ]ilates of the test or shell." 



In all other invertebrate animals, from the worms and Mollusca to the crustaceans 



