First answer:not all invertebrates  have a solid CNS.For example the phyllum Cnidaria has not CNS.This phyllum has an intraepitelium net  of neuron but here there is not ganglio,example, sea anemone; the medusae for example can to have nervous channel but here too there is not ganglio.Some authors consider the stathorabs of Scyphozoa medusae and the marginal vesicle of Hydrozoa medusae like ganglio,but these are not property ganglio.

Second answer: that invertebrates haven solid CNS is for answering with all body to one stimulus,for example  Gasteropoda,Cephalopoda,Asteroidea,etc , meanwhile the chordates haven a hollow CNS for asnswering to one stimulus only with that zone of his body that it was stimulated.So the efficient of CNS is related with the level of  organization of each taxa;the corporal organization of invertebrates is lower in relation to the environment so the answer of his CNS must be fast and efficient.The hollow CNS of chordate is fast and efficient too,because his level of corporal organization is an equilibrium with environment.

Dear, Mauricio

Thanks for your answer. It seems to me that invertebrates is in the focus of your interest, the situation which is obviously different from mine. My concern is concentrated on chordates and I should admit that I know nothing about invertebrates. I guess, the point is largely related to the  functions of the CNS. However, the question remains.

You might look into the embryological development of the respective nerve cords. For instance the chordate nerve cord derives from an invagination of the neural ectoderm in the "neurula" stage of development. The fact that the neural tube is hollow seems to be an artifact of the mechanism of formation. How does that compare with the development of the nerve cord in some particular invertebrate phylum? The Arthropoda have been particularly well studied.

Dear James,

From the anatomical point of view, the process is crystal clear during neurulation of chordates as a stage of organogenesis. As I mentioned before, I suppose that it is related to certain functions found in chordates while absent in invertebrates.

Cheers

There might NOT be any such functional difference. It might really be nothing but an artifact of the mechanics of development. On the other hand, that difference must certainly have other consequences. The architecture of the vertebrate eye with the light sensory cells ending up behind the vascular layers is one such consequence.