TUNICATA. 



1241 





method in which this is brought about is not 

 very clear ; it appears to be caused either by 

 the synchronous contraction of the individual 

 animals, causing a diminution in the general 

 calibre of the cylinder, and thus effecting a 

 faint ejaculation of water from the cavity of the 

 latter ; or by the posterior current of water from 

 the anal orifice of each animal being synchron- 

 ously ejected into the cavity of the cylinder, 

 and thus giving a motive power to the whole. 

 We have a more decided locomotion ex- 

 hibited in the Salpa, which is treated of by Mr. 

 Bishop* as a " syringograde " animal, in the 

 same category as " the Holothuria, and the 

 larvae of those insects whose progression is 

 effected by the alternate reception and expul- 

 sion of water to and from their respiratory 

 organs by an action similar to that of the 

 syringe." In Salpa the dilatation of the test 

 and its membranous lining causes the water 

 necessary for respiration and nutrition to 

 enter through the bilabiate posterior orifice, 

 which has a valve preventing the return of 

 the water by the same aperture-)-; a trans- 

 verse contraction of the body then expels the 

 water through the anterior orifice, and the 

 result is, that the animal is forced backwards, 

 being carried in an inverse direction to that of 

 the ejected water. J This retrograde motion 



* See art. MOTION, Vol. III. p. 433. 



t See figure of Salpa cristata, showing most of 

 the branchial muscles and the position of the valve 

 (the latter is ideal, not being visible externally 

 in the animal), fig. 228. p. 434. Vol. III. 



J In a communication from my friend Mr. Pittard 

 on the subject of Syringograde movement, he re- 

 marks as follows : " I am not aware that a true and 

 satisfactory explanation has ever been given of the 

 backward motion of bodies resulting from the ejec- 

 tion out of them of jets or currents of fluid. That 

 the action of such jets is at all comparable to the 

 thrusting out of a solid body, or pole, against some 

 resisting object, is by no means maintainable; for 

 the property of rigidity, which is totally and in all 

 circumstances absent 'in fluids, is an indispensable 

 condition for the production of such a result. Nor 

 is the action due to the resistance of the external 

 medium in any other way, but solely to the hy- 

 draulic pressure of the contained fluid on the in- 

 ternal surface of the walls of the hollow body. It 

 has been supposed that the resistance of the sur- 

 rounding medium acts in some degree like the walls 

 of a cannon would in the following case : If a 

 bomb-shell, containing gunpowder, in the act of 

 exploding, were placed in a cannon with the match- 

 hole of the shell open and turned towards the 

 breech, so that an explosive jet should be projected 

 into the cavity of the cannon, the shell would be 

 forced out with a much greater impetus than it 

 would have if, cteteris paribus, it were in the open 

 air. But this is not a parallel case, for here you 

 have the resistance of the closed end of the cannon 

 in one direction, and an open end, or no resistance, 

 in the opposite direction ; whereas, in the case 

 under consideration, you have just the same re- 

 sistance from the external medium in front of the 

 body moved, as you have behind it ; and whatever 

 tendency to motion might result from such a re- 

 sistance of the medium to the jet behind, just so 

 much would there be resisting motion in front ; so 

 that the effect would be neutralised, and no motion 

 would result. The truth is, that from the two 

 absolute laws of hydraulics, that a fluid presses 

 equally on all portions of the surface on which it 

 acts, and that the direction of this pressure or 



has caused the posterior orifice to be regarded 

 by some naturalists as the true oral orifice. The 

 alternate action, of dilatation and contraction, 

 have been observed to take place about fifteen 

 times in a minute ; and have sometimes been 

 termed the systole and the diastole. The 

 contraction is effected rapidly, but the relax- 

 ation, or rather the dilatation of the sub- 

 elastic test, takes place but slowly. These 

 movements are synchronous throughout a 

 chain or group of Salpce. 



After all, this is but an imperfect amount of 

 locomotion, and it is extremely prohable 

 that both the single Salpce and the Salpian 

 wreaths and chains, the latter often several } ards 

 in length, are, like the feebly moving Pyrosoma, 

 the sport of wind and wave, wafting them 

 hither and thither, either to bask calmly in the 

 sunshine, or to be broken on the rocky shore. 



AFFINITIES OF THE TUNICATA. Many 

 of the early naturalists (as we have noticed 

 in the first part of the article) noticed the 

 analogies existing between the Ascidia and 

 the Ostrea, Mytilus, and other Mollusca. 

 The compound Ascidians, however, long 

 remained grouped with Alcyonium, until Sa- 

 vigny, Lesueur, and Desmarest pointed out 

 their alliances with the simple Ascidians and 

 with Molluscs. Lesueur also demonstrated 

 the ascidian character of Pyrosoma, and re- 

 moved it from the Radiata, amongst which it 

 had been grouped ; and Cuvier pointed out 

 the alliance of Salpa with Ascidia and Mollusca. 

 Soon after the publication of the important 

 zoological conclusions thus arrived at by his 

 distinguished countrymen, Lamarck formed 

 these animals into a new group, under the ap- 

 pellation of Tuniders* (Tunicata), provision- 

 ally placing them between the JRadiata and 

 Vermes y and expressed strong doubt of their 

 general alliance with the Mollusca. 



John Hunter, who perceived the relations 



force is at right angles to such surface, 'it results 

 that the force exerted by or through a fluid con- 

 tained in a closed hollow body is exactly equal in 

 opposite directions ; that is to say, whatever may 

 be its force impelling towards the right, by just so 

 much does it impel towards the left ; 'however 

 much it impels forwards, by just so much does it 

 impel backwards ; and so on for up and down, and 

 all other opposite directions : so that the one exactly 

 antagonises the other, and an equilibrium results. 

 But remove a portion of the wall ; in other words, 

 make a hole through which the fluid may escape, 

 and then the pressure, in the direction of the side 

 in which the hole is made, is minus, or less than 

 the force in the opposite direction, by the size of the 

 hole. The body, therefore, moves in the direction 

 opposite to the hole, in obedience to the plus, or 

 excess of hydraulic pressure in that direction." The 

 Salpa, compressing the water in its interior, pro- 

 duces a large amount of hydraulic force. Its pos- 

 terior orifice being closed, it presents a hole or 

 deficiency in the front only, and is impelled in the 

 opposite direction. Also, during the act of taking 

 in water behind, the animal moves likewise back- 

 wards, for then there is, so to speak, an inversion of 

 the process described above. The pressure of the 

 external water, otherwise equal on all parts of its 

 surface, is minus behind by the size of the posterior 

 orifice, and therefore plus in front, again impelling it 

 backwards. 



* Applied at first to the BotryllidcE only. 



