AND DEVELOPMENT OF PYROSOMA. 201 



tendent of the Marine Department of the Board of Trade, kindly forwarded to me a very 

 beautiful specimen of Pyrosoma giganteum, taken by Captain Callow* in tlie North 

 Atlantic, about 400 miles S. of the Cape De Verd Islands, in the month of August of that 

 year, and admirably preserved by immersion in strong spirits. I was aware, from former 

 experience, that the textures of Ascidians, in general, are admirably conserved in spirit- 

 specimens which are even many years old; and I therefore commenced my inquiries 

 with a sanguine expectation of being able to make out something about the origin of 

 the compound embryos, which a ciu'sory inspection of the specimen proved to exist in 

 abundance. I must confess, however, that I had no anticipation that researches con- 

 ducted upon a preserved specimen of any animal could be followed out so far as I have 

 been led, step by step, to carry these. And had I not had the opportunity of showing 

 many of my preparations to observers of experience and authority, who can bear witness, 

 at any rate, to the perfect distinctness of the most important of the appearances described, 

 I should hardly have hoped to secure a patient reception for delicate embryological 

 inquiries which profess to have been conducted upon thin sections of a spii-it-specimen, 

 rendered clear by glycerine. 



I have already published a brief notice of the most important facts which have been 

 developed by my investigations in a paper published in the ' Annals of Natural History ' 

 for January 1860, and in a communication to Section D. of the Meeting of the British 

 Association at Oxford in July 1860. 



§ 2. The Anatomy o/PmosoMA giganteum. 



In the specimen of Tyrosoma under description, the ascidiariumf is a firm, hollow, 

 conical body, 4 inches long, and about |ths of an inch wide at its broad, open end, whilst 

 its rounded apex measures hardly more than half an inch. The translucent, colourless 

 wall of the ascidiarium is on an average about T^g^hs of an inch thick; but it thins 

 towards the open end, ending in a sharp ledge or rim, which is bent horizontally inwards 

 and ends in a sort of circular valve-like lip, nearly |th of an inch wide, around the 

 aperture of the central hollow or cloaca. 



In relation to the ascidiozooids, the closed, apical, end of the ascidiarium is dorsal or 

 haemal, inasmuch as the heart is situated on that side of the body of every ascidiozooid 

 which is turned towards the apex. The nervous ganglion, on the other hand, is on 

 the opposite side of the body, so that the open extremity of the ascidiarium is its neural 

 end. 



The outer surface of the ascidiarium is rendered uneven by conical eminences, which 

 are scattered over it at irregular intervals, and which are elongated on their neural sides 

 into longer or shorter processes. Among these lie similar eminences without such pro- 

 cesses and varying in elevation, until they hardly project at all above the general level of 

 the convex surface of the ascidiarium. Each of these eminences bears a small rounded 



* Finding a specimen could be procured in no other way, this gallant and skilful seaman swam for that he 

 obtained. He informs me that it emitted a strong bluish-white light, sufficient to read small print by. 



t The entire body of a compound Ascidian may be conveniently termed the ascidiarmm, while the separate 

 zooids may be called ascidiozooids. 



VOL. XXIII. 2 E 



