October 1, 1898.] 



KNOWLEDGE 



221 



the body-wall forms a mucous secretion, by which the 

 food-particles are arrested and guided into the gullet, to 

 undergo digestion in the stomach. The water, on the other 

 hand, does not take this course, but passes through the 

 tiny slits of the delicate basket-work composing the walls 

 of the pharynx. The edges of these slits are beset by 

 little lashing threads, known to biologists as cilia, and the 

 result of their rhythmic motion is that a continuous 

 current of water is driven from the cavity of the pharynx 

 to the surrounding atrium. The slits in the pharynx-wall, 

 arranged in transverse rows, are very numerous. Now, 

 between each row of slits runs a little blood-vessel, and 

 tiny branches also follow the delicate partitions between 

 the slits themselves. The walls of the blood-vessels are 

 excessively thin, and the oxygen contained in the sea-water 

 is thus able to diti'use through the walls into the blood as 

 the water swlUs through the slits. Waste carbon dioxide 

 passes out from the blood into the water at the same 

 time. Hence we have here all the essentials of a breathing- 

 process. 



The blood is constantly renewed by the beating of a little 

 heart placed ou one side of the stomach. The heart works in 

 a somewhat peculiar fashion. The contractions are for some 

 time in one direction, and then the motion is suddenly 

 reversed, the blood being propelled in the opposite direction. 

 In this manner does the adult creature live, if such an 

 uneventful existence can be called living. It spends its 

 days sedately rooted to the spot where, on abandoning 

 the wayward habits of youth, it first settled down, and its 

 obvious movements are limited to occasional contractions 

 of the outer coat or " tunic." If the animal has any 

 intelligence at all it is of the most rudimentary character, 

 and it is even problematical whether it possesses any special 

 sense-organs. There is a mass of nervous matter just at 

 the beginning of the pharynx, and this and some neigh- 

 bouring structures may be of use for testing the quality of 

 the water flowing in at the mouth, but organs of sight and 

 hearing are quite absent. The life of an oyster is in com- 

 parison one of pleasing variety. 



It is one of the greatest triumphs of the still young 

 science of embryology to have shown conclusively that 

 this creature — little more than an automaton, and possess- 

 ing no obvious trace of vertebrate structure — is yet a 

 member of the great sub-kingdom to which all birds, 

 mammals, reptiles, amphibians, and fishes belong, and of 

 which we are pleased to consider ourselves the crowning 

 pinnacle and glory. A brief icsumc of the features which 

 biologists consider to be essential characters of vertebrate 

 animals may assist the reader to a better appreciation of 

 the masterly piece of research by which Kowalewsky showed 

 the Ascidian to be a fallen vertebrate, and gave to it a 

 position of quite unique interest. 



In the first place, all vertebrates possess a supporting 

 skeletal rod running al«ng the main axis of the body. 

 This is usually the " backbone," but it may be represented 

 by a spinal column of cartilage or gristle, as is the case 

 with the sharks and their aUies. In the lowest vertebrates, 

 and in the embryos of all the higher ones, the skeletal axis 

 consists of a simple continuous rod called the notochurd, 

 which is of the consistency of stiff jelly. Secondly, the 

 central nervous system of aU vertebrates arises as a groove 

 along the middle line of the back or " dorsal " surface of 

 the developing animal. The edges of the groove arch over 

 and meet, converting it into a tube, which becomes the 

 spinal cord and brain. Moreover — and this is a fact of very 

 great interest to the evolutionist — every member of our 

 great sub-kingdom passes through a stage in which the 

 pharynx (already defined as the part of the digestive tube 

 immediately following the mouth) has its side-walls per- 



forated by slits. These gill-slits are present throughout 

 the whole life of fishes. The water taken in at the mouth 

 escapes through the slits, a. rating the blood flowing through 

 the gills on the margins of the slits as it does so. Am- 

 phibians, which nearly all spend their infancy m water, 

 breathe during the greater part of their aquatic life exactly 

 as do the fishes. When, however, the tadpole attains his 

 froghood and leaves the water, his gill-slits close, and he 

 breathes by lungs. This early habit of water-breathing 

 probably indicates that frogs are descended from fish-like 

 ancestors, and that the tadpole repeats, to some extent, his 

 ancestral history in his own development, or, as Marshall 

 happily expressed it, cUmbs up his own genealogical tree. 

 Again, every bird and reptile, whilst in the egg, passes 

 through a stage with gUl-slits piercing the sides of the neck, 

 slits which are of no conceivable use to it as organs of 

 respiration, and which are only explicable as ancestral 

 features which have persisted through countless ages. 



It IS clear, then, that no animal can justly claim the 

 proud title of vertebrate unless it possess at some period 

 of its existence ( a) a notochord, i i) a dorsal tubular nervous 

 system, (c) gill-slits in the wall of the pharynx ; and 

 Kowalewsky's famous research showed that the sea-squirt 

 passes through a stage in which all three are present. 



As he watched the tiny egg develop, he saw the single 

 cell divide up until a hollow two-layered ball of cells was 

 formed. The cavity of the ball, the primitive digestive 

 sac, communicated with the exterior by a small pore. 

 Next one side of the ball became flattenetl and then 

 grooved. The groove was bounded by right and left folds, 

 which soon began to arch over and unite at the hinder 

 end. The union extended farther and farther forward on 

 the dorsal surface until a tube was formed, the rudiment 

 of the spinal cord and brain. 



That a mollusc, as the sea-squirt was supposed to be, 

 should develop a hollow nervous system in this manner 

 was a very remarkable circumstance, and we can imagine 

 with what breathless interest the observer must have 

 watched the further growth of the little embryo. For what 

 followed was stranger stiU. A rod of cells between the 

 nerve-tube and the digestive sac became more and more 

 prominent, and soon acquired all the characteristics of a 

 veritable notochord. Then the hinder part of the embryo 

 began to grow out as a tail, carrying both spinal cord and 

 notochord with it. At the opposite end a mouth opened 

 into the digestive tube, and the enlarged front end of the 

 spinal cord developed an eye and an organ of hearing. The 

 embryo was now a free-swimming larva, which was in 

 appearance and structure curiously suggestive of a tadpole, 

 but it was of very minute size. Openings soon perforated 

 the walls of the pharynx-region, but the growth of an 

 atrium round the pharynx shortly afterwards shut off 

 these gUl-slits from communicating directly with the 

 exterior. 



The tiny larva, which thus conformed completely with 

 vertebrate requirements, swam about vigorously for a 

 few hours by means of its fish-like tail-fin, and then — " ! 

 what a fall was there ! "—it fixed itself by some little 

 suckers which had appeared under the mouth, the tail 

 grew less and less, and eventually vanished altogether, 

 taking notochord and spinal cord with it ; the eye and the 

 organ of hearing disappeared ; and the front end of the 

 nerve tube, too, so hopefully suggestive of a brain, dwindled 

 until nothing remained but a little shapeless mass. Gone, 

 " like the baseless fabric of a vision," were all vertebrate 

 characters save a few poor gill-slits. These slits increased 

 in number, various changes in the relative size of other 

 organs occurred, and the animal stood revealed, a prosaic 

 and phlegmatic sea-squirt. 



