SEA-SQUIRTS— ACORN- WORM, ETC. 369 



lantern-like breathing chamber, which, except for one line of attachment, hangs 

 freely within the coat situated just inside the outer bag. 



The above-mentioned Ascidia mentula, which represents the simple type of 

 tunicate structure, is hermaphrodite, and produces floating eggs, which ultimately 

 develop into the free-swimming tadpole-like larvte to which reference has already 

 been made. Ascidia mammiUata, the species represented in the accompanying 

 illustration, is another member of the same section. Compound ascidians consist of 

 a number of minute sea-squirts aggregated together into a gelatinous mass which 

 encrusts seaweeds or rocks ; Botryllus violaceus being an example of this section. 

 In some cases the units of such a colony are joined into groups, the discharging 

 orifices of all the members of which coalesce to form a common outlet. Strangest 

 of all are the hollow, cylindrical colonies of the genus Pyrosoma, in which all the 

 inhalent apertures are arranged on the outer and the exhalent ones on the interior 

 surface ; these apertures in this instance being situated at the two poles of the 

 constituent individuals instead of being placed near together. The whole cylinder 

 is open at one end and closed and truncated at the opposite extremity. These 

 ascidians, which are represented by about half a dozen species, varying in length 

 from a few inches to as much as four feet, are, as their name (pyrosoma = fire body) 

 implies, brilliantly luminous at night, emitting a green light, which may be visible 

 for miles in regions where the sea is swarming with them. 



Lastly come the free-swimming forms represented by the barrel-shaped types 

 known as Salpa and Doliolum and their allies. Salpas are so transparent that 

 they can seldom be detected in waters where they are present in multitudes. They 

 swim in jerks, taking in at each inspiration numbers of the minute organisms 

 constituting the so-called plankton of the ocean surface, which form their nutriment. 

 The most remarkable peculiarity connected with their life-history is that salpas 

 exhibit the phenomenon of alternation of generations, that is to say, a certain indi- 

 vidual resembles its grandparents, while individuals of the intermediate generation 

 are of quite a different type and like the great-grandparents of the first individual. 



Another group on the borderland between vertebrates and in- 

 Acorn-Worm, etc. . ° . • 



vertebrates is typified by the acorn-worm (BaLanogLossios), an organism 



living buried in the sand or mud of the seashore, and exhaling a strong odour of 



iodoform. It secretes a sticky slime to which particles of sand and shell adhere, 



thereby forming a protective tube. A long and sensitive retractile proboscis, 



somewhat recalling in appearance the flower-rod of the common wild arum, and 



of a bright orange-yellow colour, forms the upper end of the animal, behind which 



is a kind of collar, overlapping the proboscis in front, and part of the slits of the 



gills behind, and also concealing the mouth. Behind this, again, comes the proper 



body of the animal, invested in its protecting tube, which may be as much as a 



yard in length. To describe this is unnecessary on this occasion, and it will 



suffice to mention that the gill-slits present a striking resemblance to those of the 



lancelets (to which reference has been made in an earlier chapter), and that it 



also contains structures corresponding to the notochord and the nerve-tube of 



vertebrates. The acorn-worm is a native of the northern seas, but a second 



species of the genus occurs in New Zealand. An allied genus, Harrimania, 



inhabits the coasts of Alaska, and is considered to be the most primitive member 



vol. in. — 24 



