Oct. 5, 1882J 



NA TURE 



563 



very far, but as Prof. Lutken 1 has shown the degree of deve- 

 lopment of the pectoral fins bears no constant relation to the 

 >ize attained by the young fisb, a great increase in size may occur 

 without a corresponding progress in metamorphosis. In con- 

 sequence of this the young of the common flying Gurnet 

 Dactyloptaus volitans, were not for long recognised as such but 

 were considered distinct and named Cephalacanthus. A 

 parallel instance to that of Leptocephalus is possibly that of the 

 curious flattened larva of the Kock Lobster (Palinurus) 

 Phyllosoma, which is also found in the open ocean attaining 

 sometimes gigantic proportions. Possibly also other pelagic 

 larvae become thus hypertrophied in the larval condition. We 

 may compare with these phenomena the somewhat parallel 

 modifications which occur naturally or may be produced arti- 

 fically amongst larval Amphibians. 



Many of the Pelagic animals carry with them parasites 

 similar to those affecting their littoral allies and which thus 

 are, as it were, imported into the Pelagic fauna, tut there are a 

 few definitely pelagic parasites parasitic upon pelagic hosts. 

 The young of the Pelagic annelid Alciopa are parasitic within 

 the bodies of Ctenophora, there is the small parasitic Hydro- 

 medusa Mnestra, which adheres to Phyllirhoe, and lastly there 

 are the young Cunina medu-as which cling in den'e clusters 

 within the stomach of the Geryonid Medusa Carmarina, and 

 were at first imagined to he the young of the Carmarina itself. 



A remarkable feature about Pelagic animals is that very many 

 of them occur in large swarms, some in immense hosts. Further 

 Velellas, Porpites, and Ianthh.es are always met with in schools, 

 and even Leptocephali, and very many other forms are usually 

 caught in the tow-net, several at a time. 



In their almost univer-al geographical distribution except as 

 regards the colder seas, Pelagic animals resemble the deep-sea 

 fauna ; as examples it may be mentioned that according to Prof 

 Lutken, the tunny of the Mediterranean is identical with that of 

 Japan, and the albacore of the Atlantic with that of the Pacific. 

 Pelagic genera seem to be of almost ubiquitous distribution, 

 though the Atlantic and Pacific species frequently differ. 



Some few Pelagic forms seem to be remarkably scarce. As 

 an instance may be cited Pelagonemertes, the curious Pelagic 

 Nemertine with a ramified intestine. This form was obtained 

 in great abundance by Lesson at the surface in 1830, between 

 the Moluccas and New Guinea. By the Challengir it w as found 

 twice, only a single specimen being got on each occasion. The 

 first was caught to the south of Australia, and the second on 

 the coast of Japan. The animal seems never to have been met 

 with by any one excepting on these three occasions. On each 

 occasion when caught by the Challettger it was found in a trawl 

 which had been down to a great depth. It is therefore very 

 possible that it very rarely rises to the surface. 



Similarly many Pelagic Cephalopods though known to exist 

 in multitudes are of the greatest rarity, being only known from 

 fragments. Bushels of their horny beaks are found in the 

 stomachs of whales, which subsist on them, and several genera 

 are known to Frof. Steenstrupp only from these quantities of 

 beaks. He has never seen a trace of any other part of them. 



Notwithstanding the wide distribution of Pelagic forms, Mr. 

 Murray finds that he is able to form tolerably correct conclusions 

 as to the latitude of any sample of deep-sea bottom which con- 

 tains organic remains submitted to him, from the nature of the 

 Pelagic debris of w hich it is compesed. He can also form some 

 idea of the depth from which a deposit has been brought up by 

 observing the extent to which the substance of the calcareous 

 shells has undergone solution. Pteropod shells owing to their 

 extreme thinness appear to be dissolved first, and disappear say 

 at I2CO fathoms, then the finest globigenna shells at 2200, 

 then the larger globigerina shells and so on. 



Pelagic animals as a rule appear to be extiemely sensitive to any 

 lack of saltness in the water. The surface fauna of the Baltic 

 is thus very poor, and in the upper part consists of little else 

 than a few small Crustacea, but curiously enough the large 

 Scyphomtdu-a;, such as Aurelia and Cyanea appear to be un- 

 affected injuriously by a brackishness of the water but rather to 

 prefer it. They extend in the Baltic into places where the water 

 is very little salt and I have seen similar large Scyphomedusae 

 swimming in shoals at the head of one of the large creeks of 

 the Hawkesbury inlet in New South Wales, in the actual current 

 of a small fresh-water stream which ran in and where the water 

 v as quite drinkable. This is all the more remarkable because 

 as Mr. Romanes has shown the one Hydromedusa w hich we know 



1 Ch. Lutkeo, Spolia Atlantica. Copenhagen, 18S0, p, 426. 



of as confined to fresh water, the well known Lymnocodium of 

 the Victoria lily tank in the Regent's Park Botanical Gardens, 

 is excessively sensitive to any addition of salt to the water in 

 which it is. 



I am informed by Mr. George Baden Powell that the large 

 Medusae so abundant here at Southampton, shows a curious 

 tendency to crowd up towards the higher part of Southampton 

 water. There are hardly any to be found as a rule in the Solent 

 but they appear always to tend to crowd up at the heads of 

 estuaries. I have noticed in Norw ay also that they appear to 

 crow d at the heads of the Fjords. 



I shall now proceed to some remarks on the zoological com- 

 position of the Pelagic fauna and its probable history in the 

 past. The present Pelagic fauna may be regarded as consisting 

 of two constitutents, firstly, a number of species belonging to 

 a series of orders and subclasses which are absolutely peculirr 

 to it, that is to say, which have no representatives which are 

 littoral or terrestrial, and are not at any period of their exist- 

 ence other than Pelagic. We may reckon about nine such 

 groups. There is no group which rises undoubtedly to the rank 

 of a class w hich is thus Pelagic only. The groups are as follow: , 

 the Siphonophora, Ctenophora,' Chaetognatha, Heteropoda, 

 Pteropoda, Larvalia, Salpae, Pyrosomidct, Cetacea. 



Of the antiquity of the Siphonophora we know nothing 

 directly, for they do not occur at all as fossils, and as they are like, 

 most pelagic forms ill adapted for preservation as fossils, it is 

 impossible to conjecture whether ihey are of quite modern or ol 

 ancient origin. They are complex colonies of animals of various 

 forms united together and performing separate functions for the 

 common good of the colony. They are offsets of the Hydrome- 

 dusae, and thus derived originally from a pelagic planula ancestor, 

 but it seems uncertain whether they have subsequently sprung 

 from a once fixed hydromedusa stock set free, or have been free 

 and pelagic throughout their history. The Ctenophora are also 

 an offset from the Hydromedusa;; they also have as yet no 

 geological history. Their ancestors have ] robably always from 

 the planula upwards led a free pelagic life. The histoiy of the 

 Cha;tcgnatha (Sagitta) is obscure. The Heteropods and Ptero- 

 peds are derived from a common pelagic veliger ancestor which 

 existed as early at least as Silurian times, and this ancestor 

 probably descended from a trochosphere also pelagic. 



The Larvalia, the Ascidian Appehdicularia and its allies, near 

 relatives of the ancestral vertebrata, probably have always been 

 Pelagic and have existed in something like their present form 

 from a very early period, whilst the Pyrosomidce after branching 

 off from the same stock as simple animals have possibly under- 

 gone a fixed sessile condition as compounds before becoming 

 again Pelagic. 



If Prof. Ray Lankester is correct in his suggestion (in 

 his British Association lecture on "Degeneration") that very 

 possibly an ancestor of all the vertebrata, including man himself 

 was once pelagic, because the peculiar mode of the develop- 

 ment of the eye of veitebrates can only be accounted for by the 

 supposition that the tissues of the head were completely trans- 

 parent and from other considerations ; then the whales ere now 

 so to spieak for the second time pelagic in the histoiy of life. 

 Their more immediate ancestors, allies of- the seals, and sprung 

 from the common progenitor of the stock of placental mammals, 

 took afresh to the sea and gradually relinquished the shore 

 altogether. 



The second division of the Pelagic Luna i? composed of 

 numerous representatives of various classes and orders of 

 animals, the majority of members of which are inhabitants of the 

 sea bottoms, shores or land surfaces, but w hich representatives 

 are mostly specially modified in remarkable ways to fit them for 

 pelagic existence. Only a few- of these can now be touched ( n. 

 Although there are abundance of Cilioflagellata which are 

 pelagic, there seem to be very few true Infus oria (or Ciliata) which 

 are so, at least very few have as yet been recoided as such, and 

 none at all known from any great distances from land. The few 

 as yet known all belong to one family of the Peritricha, the 

 TintinnidK. Codonella, one of ihem, of which a representation 

 is now on the screen, is bell-shaped and remarkable for being 

 provided w ith a siliceous protecting shell. 



There are even sea anemonies which have taken to Pelagic 

 existence, and are to be found in great quantities en the ocean 

 surface at times. They are exactly like the ordinary sea 

 anemonies of our shores, exceptii g that their base instead of 



* It is possible as suggested by the late Pro f. Balfour, that Kewalensky's 

 CoeK plana may prove to be a creeping Ctenophor. 



