78 REPORT — 1851. 



arranged. At first siglit it seems to present equally strong affinities with four 

 principal groups, viz. — 1. tlie Nematoid worms; 2. the Annelida; 3. the Lernaean 

 Crustacea; and 1. the Arachnida. 



1. With the Nematoid worms it is allied hy its general shape and hahit, its want 

 of distinct annulation, and remotely, by the armature of the mouth. But on the 

 other hand, it differs widely from them in the nervous system, the sexual system, and 

 the nature of the muscular tissue. 



2. Sayittci has no small resemblance to certain Naiadae, in which when young the 

 anterior hook-like feet are directed forwards parallel to the mouth. It differs from 

 them in the nature of its nervous system, which exhibits a concentration quite foreign 

 to the annelid type, in the nature of the muscular tissue, and in the total absence of 

 any water vascular system. 



3. and 4. The real affinities of Sagitla are probably with one or other of these great 

 divisions. The structure of the nervous and muscular system speaks strongly for 

 this view, and the nature of the sexual system is not opposed to it, inasmuch as we 

 have hermaphrodism among both the lowest Crustacea (Cirrhipedia) and the lowest 

 Arachnida (Tardigrada). 



The study of development can alone decide to which of these divisions Sagitta 

 belongs; but until such study shall have demonstrated the contrary, Mr. Huxley 

 stated his belief that Sagitta bears the same relation to the Tardigrada and Acaridae, 

 that Linguatula (as has been shown by Van Beneden) bears to the gewws Jnchorella, 

 and that the young Sagitta will therefore very possibly be found to resemble one of 

 the Tardigrada, the rudimentary feet with their hooks being subsequently thrown up 

 to the region of the head, as they are in Linguatula. 



An Account of Researches into the Anatomy of the Hydrostatic Acalephee. 

 By Thomas H. Huxley, F.R.S., Assistant Surgeon R.N. 



The observations upon which this communication is based were made during the 

 circumnavigatory voyage of H.M.S. Rattlesnake, but for the most part in the seas 

 which border the coasts of North-eastern Australia, New Guinea, and the Louisiade 

 archipelago. 



With the exception of the mere external form, but very little has been known 

 hitherto with regard to either the Diphydse or the Physophoridae, the two families of 

 which the ' Hydrostatic Acalephae' of Cuvier consist, altlio\igh they are some of the 

 most abundant of pelagic creatures. Indeed, hardly any one can have made a voyage 

 to the East Indies or Australia without being struck with the immense shoals of the 

 Phgsalia and Velella, through which the ship sometimes s ils for days together. 



The chief mass of one of the Diphydae is formed by two transparent crystalline 

 pieces, which look, when taken out of the water, like morsels of cut glass. One or 

 both of these pieces contains a wide cavity, lined by a muscular membrane, by the 

 contraction of which the animal is propelled through the water. The attachment of 

 the posterior piece to the anterior is very slight, and when detached it will swim about 

 independently for hours together. It was this circumstance which led Cuvier to 

 consider the two pieces as two distinct animals. 



In the Monogastric Diphydse a single polype is developed in a special cavity of the 

 anterior piece. In the Polygastric DiphydiB, a long chain of such polypes, each en- 

 veloped in a little transparent " bract," occupies a similar position. These polypes 

 have no oral tentacles, but a long thread-like tentacle, bearing lateral branches, which 

 are terminated by small sacs, is developed from the base of every polype. . The small 

 " prehensile" sac has a vei-y peculiar form, but is, morphologically, only a dilatation 

 of its pedicle, one wall of which is much thickened, and contains a great number of 

 such urticating organs or "thread-cells" as are found among the Medusae. The re- 

 productive organs are medusiform bodies which are developed by gemmation from the 

 pedicle of the polype. 



The central sac of the medusiform body, instead of becoming a stomach, developes 

 the spermatozoa or ova within its walls. These are generally shed forth while the 

 organ is still attached, but in one genus they swim about independently, and might 

 readily be mistaken for Medusae. 



In the Polygastric Diphydsae new polypes are continually being produced by gem- 



