Dec. 27, 



NATURE 



201 



distribution of the minute spicules throughout the colonies 

 of Leptoclinum and some other Compound Ascidians, I 

 know from experience that they affect the hardness and 

 roughness, as well as the colour, of the colony, and so 

 may be of considerable importance in repelling enemies 

 and in keeping the colony free from injurious parasites. 

 As a matter of observation, I find that the colonies of 

 Didemnidie (which are provided with calcareous spicules) 

 are much freer from both external and internal parasites 

 than are the softer-tested Compound Ascidians. 



During the last few years I have had occasion to study 

 closely the fauna of the sea-shore at different parts of our 

 coast. I have spent many hours on the rocks at Puffin 

 Island and elsewhere at extreme low water, watching the 

 animals in the pools and under the ledges in their natural 

 conditions. Such work impresses very forcibly upon the 

 observer the reality and importance of such fundamentals 

 of evolution as variation, over-crowding, and struggle for 

 existence, the action of natural selection, the benefit of 

 protective colouring, the completeness and the advantage 

 of mimicry, the benefit of spicules, shells, various shapes, 

 &c., the purposes and origins of peculiar habits, the com- 

 plicated relations between the animals and their environ- 

 ment, and finally the utility of specific characteristics. 



Of all the regions of sea and land, so far as my limited 

 experience goes, by far the most prolific of animal life is 

 that region of the shore which is known as the upper 

 edge of the Laminarian zone. It lies just beyond the 

 ordinary beach, and is only exposed at the lowest spring 

 tides. There, amongst the tangled masses of Laminaria, 

 especially if there are large irregular stones with many 

 pools and crevices between, marine invertebrate life is to 

 be seen in very great profusion ; in a favourable locality, 

 such as Puffin Island, all the chief groups of marine 

 animals being abundantly represented. There, competi- 

 tion is probably very keen amongst allied forms of 

 animals, and the conditions necessary for natural selection 

 to take place, and the results of that process, may most 

 advantageously be studied. 



I have lately been noting which animals in this 

 region of the shore appeared to be the most con- 

 spicuous by their colour or shape or other peculiari- 

 ties on various occasions and in different localities, 

 with the object of seeing how far the want of pro- 

 tective colouring or attempt at concealment can be 

 accounted for ; and the result is that, so far as I have 

 observed, all the most brightly-coloured or otherwise very 

 noticeable species are provided with some defensive or 

 offensive contrivance which appears to protect them from 

 enemies. Amongst these conspicuous forms are : the 

 white calcareous and some of the scarlet and other 

 brightly-coloured siliceous Sponges (which are well pro- 

 tected by their numerous pointed spicules), the gleaming 

 white Compound Ascidians (provided with sharp-pointed 

 calcareous spicules), some of the Annelids (protected by 

 their setx, elytra, &c.), some of the Polyzoa, such as the 

 Escharidae and other incrusting forms (covered by cal- 

 careous ectocysts, often provided with spines and other 

 projections), and a few erect forms such as Bugula (which 

 are protected by the presence of numerous sharp-beaked 

 snapping aviculatia). The bright orange-coloured Bugula 

 Uirbinata is certainly one of the most conspicuous 

 animals on certain parts of the shore about low-water 

 mark at Puffin Island. Of course such protective cha- 

 pters as these animals possess are not all necessarily 

 [lecific, but may be generic, or characteristic of still larger 

 ^loups, and I am not now citing them as instances of 

 useful specific characters, although I do not doubt they 

 would prove such, if the details were properly worked 

 out. 



As examples of these last (useful specific characters), I 

 would pomt to the distinctive features of the species of 

 Ascidians, where even the very varied external shapes may 



be regarded as useful modifications, since they allow of, 

 or correspond to, particular forms of the muscular mantle 

 and the branchial sac a'nd the other viscera within the 

 test, and of course the shapes of the mantle and branchial 

 sac are of functional importance. It is important to note 

 that the one external feature which it is difficult to see 

 any use in— viz. the number of lobes surrounding the 

 branchial and atrial apertures — is not a specific character, 

 but is distinctive of genera, and even more so of families 

 and sub-families. 



On passing to the interior of the body in the Ascidians, 

 we find that the best-marked characters are taken from 

 the condition of the mantle and of the branchial sac, and 

 its neighbouring structures the tentacles and the dorsal 

 lamina. Now, these are all organs with most important 

 functions to perform in regard to respiration, nutrition, 

 the circulation of water through the body, and the collec- 

 tion and agglutination of food-particles. And all the struc- 

 tural modifications found are such as must evidently be of 

 actual use to the possessors. The current of water pass- 

 ing through the body of an Ascidian— in at the branchial 

 aperture, through the stigmata in the walls of the branchial 

 sac into the atrial cavity, and from that out by the atrial 

 aperture — is of primary importance, since it serves the 

 following purposes : (i) it conveys oxygen into the body 

 for respiratory purposes, (2) it brings the food-matters 

 into the body, (3) it removes waste matters from the body, 

 and (4) it conveys to the exterior the ova and spermato- 

 zoa. This current of water is caused and guided by {a) 

 the shape of the mantle and the arrangement of the 

 sphincters and other muscles, and {b) the cilia covering 

 certain of the vessels and other parts of the wall of the 

 branchial sac. Hence modifications of the form of 

 the mantle and of its muscles, and of the vessels, bars, 

 papillae, &c., forming the wall of the branchial sac (which 

 are precisely the characters made use of in distinguishing 

 the species), must surely be of functional importance, or, 

 in other words, are useful modifications, such as would be 

 produced by the action of natural selection. 



It is scarcely necessary to call attention to such important 

 adaptive characters as the arrangement of the blood-vessels 

 and water-passages in the walls of the branchial sac, but 

 it may be pointed out that even such trivial structures as 



I the spine-like scales lining the branchial siphon in some 

 Cynthiidae may well be more or less useful, according to 



I their shape and size, in keeping out small unwelcome 

 intruders, such as the young of the parasitic Copepoda, 



I sometimes found in the branchial sacs of some Ascidians. 

 Another point in which species of Ascidians differ is 



' the condition of the tentacles round the entrance to the 

 branchial sac — i.e. their number, shape, branches, and 



i arrangement. These organs probably perform various 

 functions : they break up and distribute the currents of 



I water, they intercept and guide the food-particles, they 



! probably act as sensory organs, and they form a more or 

 less perfect grid for preventing large objects from entering 

 the branchial sac. Hence there can be little or no doubt 



i that in this case also the various modifications are really 



! useful.^ 



These few instances are, perhaps, sufficient to show 



1 that, in the Tunicata at least, specific characters are of 

 actual importance to their possessors, and are adaptive 



I modifications such as would be produced by the action 



j of natural selection ; and I fancy that the same will be 

 found to be the case in other groups of animals, if those 



I biologists who are intimately acquainted not only with 



, the characters of the various species, but also with their 



, habits in a condition of nature, and the environment 

 generally, would turn their attention to the matter. 



W. A. Herdman. 



' I am working up the matter in further deta.l for the Repo-t upon the 

 Tunicata in ihe second volume of the " Fauna of Liverpo.l Bay." 



