28 THE POPULAR SCIENCE MONTHLY. 



is guarded by sensitive tentacula and a sphincter muscle. The mouth 

 is situated at the bottom of this branchial sac, down the side of which 

 minute particles of food are swept by ciliary action, so as to be 

 brought within the simple commencement of the oesophagus. The 

 effete sea-water passes through the walls of this branchial cavity into 

 a general body-chamber, in which the viscera are contained. This 

 cavity is bounded externally by a muscular expansion, lining the outer 

 cellulose tunic. By the periodical contraction of this muscular sac, 

 the water which enters it, together with food-residues and ova, is ex- 

 pelled through another funnel-like opening, adjacent to and very 

 similar to that by which it gains entry to the branchial chamber. 



Although these ascidians have a definite alimentary canal, a circu- 

 latory system, and respiratory organs, together with a distinct genital 

 apparatus, their life of relation with the external world is of the sim- 

 plest description. They are stationary creatures, and have no pre- 

 hensile organs, food being brought to the commencement of their 

 alimentary canal by ciliary action. 



In correspondence with such a simple mode of life, we might ex- 

 pect to find a very rudimentary nervous system, and this expectation 

 is fully realized. The Tunicata possess a single small nervous gan- 

 glion lying between the bases of the two funnels through which water 

 is taken in and discharged. This ganglion receives branches from the 

 tentacula guarding the orifice of the oral funnel, and possibly from 

 the branchial chamber, while it gives off outgoing, filaments to the 

 various parts of the muscular sac, and perhaps to the alimentary 

 canal, and some of the other internal organs. In some of the solitary 

 Tunicata a rudimentary visual function is presumed to exist. At all 

 events, pigment-spots are situated on, or in very close relation with, 

 the solitary ganglion. This single body seems to serve for the per- 

 formance, in a rudimentary manner, of the various functions dis- 

 charged by at least two pairs of ganglia in a large number of higher 

 Mollusca, viz., those known as the cerebral and the parieto-splanchnic 

 or branchial. 



The brachiopods are among the oldest and most wide-spread of 

 the forms of life in the fossil state, and the geographical distribution 

 of their living representatives at the present day is also very wide. 

 Like the Tunicata, they are headless organisms, and lead a sedentary 

 existence, attached either by a pedicle or by one division of their 

 bivalve shells. The mouth is unprovided with any appendages for 

 grasping food nutritive particles being brought to it by means of 

 ciliary currents. Numerous muscles exist which connect the valves 

 of the shell to one another, and with the inclosed animal. And, 

 though the visceral organization of the brachiopods is somewhat 

 complex, no definite sense-organs have yet been detected in any of 

 them. In the nervous system of these sedentary animals, there is, 

 therefore, nothing answering to a brain as it is ordinarily constituted, 



